Bike Consideration .. to change or not to change

So I've been doing Tri's for a little over a year now.  I'm riding a Specialized Elite (I guess upper end of entry level bikes for specialized).  I really enjoy riding and have my bike set up so I'm pretty comfortable even on longer rides.  Part of that is likely my body has changed as I have dropped weight.  But I had a professional fitting and overall I like my bike.  Nothing too fancy and it is suiting my purpose.  I'm totally a back of the packer and just enjoy participating in tris.  Signing up gives me a goal and forces me to remain committed to training.  I'm goal oriented no doubt.  Without a goal I'd find myself back on the couch. 

So, when does it make sense to go to a nicer/upgraded bike (carbon frame, etc.)?  I probably won't go the tri bike route just because I'm about comfort.  So will likely stick with a road bike configuration.  I have shimano 105 group on my current bike and am pretty pleased with everything really.  So for me, is there any real advantage to moving to a carbon frame/etc?

I appreciate any feedback any of the bike experts have.

thanks,

Bruce