Upgrade components?

I think for your current goals (sprints/oly), the bike is fine as is.  You can take the summer to make sure you like triathlons without spending any more money on the bike.  The bike I started on wasn't much different, though it did have the 105 groupset.

I don't think you'll see much of a bike split improvement by upgrading to 105s.  What you will see is smoother shifting, more reliability, and tuning that stays consistent.  The weight difference gained going to 105 wouldn't be much.  Time saved on the bike is gained through fitness and time in the saddle, then equipment upgrades like carbon fiber frames, aero wheels, aero helmets, TT bikes, etc.

If it was me (and in light of your IM goals), I would ride what you have this summer, save your money, and keep an eye out for a used or lower end new TT bike with better components.  The 105 groupset is fine but Ultegra is a great price point and can be had in some form on most lower end tri bikes.  Again, there's no reason why you can't ride your current bike all the way through even an IM.  But rather than upgrading components on a low end bike, you can get more bang for your buck getting a new setup.  I've bought all of my bikes used off Craigslist/Ebay.  Just have to know what you're looking for. 

Good luck!

You can probably find a new bike with 105 for around 1200, much less if you look used.  If you could sell your current bike for $500, and get a new bike with 105 for $1200, you're out $700 total.  You say you can get the 105 groupset for $500-600, but I imagine you'll also need to pay for installation...which means it's likely a wash at $700.  You could try to sell your 2300 components, but chances are they will hold very little to no value on the used market.  Not many people are looking for those type of parts, and if they are, it is highly unlikely they will buy the whole set...maybe one piece here or there if they stuff they had broke.  You might get lucky if someone is looking to build up an old frame for a commuter bike or something...but don't hold your breath.

All that being said...is it worth it to move to 105 for your purposes?  Well, that depends.  105 will give you 4 extra gear combinations...with probably 3 of them "usable."  That can make a difference when it comes to riding at a preferred cadence in certain situations.  105 will shift more reliably and be more durable...but that doesn't really make you "faster."  2300 just requires more frequent tuning and you can't shift under as much load.

Also, if you do decide to swap out components instead of getting a completely different bike already built up with 105, you might want to make sure your current rear wheel is 10 speed compatible.  I'm not really familiar with what type of hubs or freewheel bodies 8 speed bikes come with, but it's certainly something you want to check out first. 

Use the bike you have and don't put more money into an entry level bike upgrading parts . You the engine is what needs to be improved.

If you want to get a better save your money in a new bike fund, and decide if you want to buy better bike or a nicer used bike instead of upgrading your group.

Coming from a biking back ground I understand what I like to call "component" envy.   Cycling is one of those sports that you really wish you could just borrow gear for a year to see if you like it because if the entry level equipment is expensive and if you like it the entry level just isn't going to work any more

Given your goal of IMTX in 2014 I would save the money and upgrade the entire bike.   I also would not shy away from used every thing even piece by piece.   As an example I just built up IMO a killer TT bike for a fraction of the cost to buy an equal bike new or used.  It took me over 4 months to assemble the pieces but I got some unbelievable deals by looking in the classified and making offers and trading stuff.  You just have to get creative.  

However, I understand the desire for better "riding" stuff.  A nice smooth light bike is a great placebo that makes you ride faster and also makes you want to build the engine to justify the bike.  To answer your specific questions.  At this point upgraded components will not not improve your bike split unless it is because of the placebo effect.   Cost to swap components at a bike shop should be something like $100-150 depending on what they have to do.  What you need to know about your components and upgrading is there isn't much mix in matching you can do because the entire drive train and shifters will have to be changed because it is 8-spd and the upgrades worth doing are 10 or 11 spd and are not cross compatible.  Once you get in the 10 spd range you would have the ability to get slowing swap out components (i.e. you could switch to a dura-ace rear derailleur without switching any of the 105 components because they are indexed the same) but you don't have that luxury with your current set up (it would require at a minimum new cranks, rings, front and rear derailleur, cassette (your wheels hub may not be compatible also), sti shifters (which may then require new brakes), etc.   That said, I think you could find a used group for less than new.  You are starting to see a shift from 10 to 11 spd and people are starting to sell their 10spd stuff.   There can be some real bargains on stuff like Dura-Ace 7800 (first 10 spd DA) or even 7900.  

I still recommend sticking with what you have and then upgrading everything late summer early fall when everyone starts thinking about getting a new ride for next year and wants to sell their current ride.