Bike for half ironman

The single biggest factor in your performance is going to be the strength of your aerobic engine.  Which bike you are on isn't going to change that.  Aerodynamic drag is the largest force you need to overcome in order to go fast and your body is the biggest source of that drag.  A TT bike will put your body in a better position and help reduce the drag.  I tell all the athletes I work with, if you are seriously in contention for a podium, Kona qualification, etc., then you need to do what all the other people in contention for those spots are doing - which is riding a TT bike.  Beyond being more aerodynamic - thus faster at the same power - the geometry of a properly fit triathlon bike will help save your legs for the run.  So it isn't just about the bike, it's also about setting yourself up for a better run.

If you are largely racing against yourself - meaning you don't have designs on a podium, etc., you can pursue PR's using a road bike w/clip-ons - you don't "NEED" a TT bike.  Except yeah, they look cool.  You said you've been doing this for awhile so you know what you're into.  If you expect you'll be doing tri's for many more years, than sure, a TT bike makes sense.  If you decide to get a tri-bike you DON'T need a ridiculously priced Super-bike.  It was arguably proven last year through an independent wind tunnel test that the Super-bikes are only a few watts faster than the mid-priced tri-bikes.

Bottom line, yes, you'll be faster on a PROPERLY fit triathlon bike.  That said, the engine will ALWAYS be more important.  You can put Sebastian Kienle on a straight up road bike and he'll beat 98% of the people in ANY race on all their fancy TT bikes.

Hope that helps.