More bike advice

paul_h5

Hi I'm after some bike advice, I've done standard / Sprint distance tris for a couple of years now and going to step up to a 70.3 next year and full iron man in 2015. I don't have loads of cash to spend on a bike so have used the halfords (unfortunately) cycle to work scheme and I'm thinking of doing it again next year.

I currently ride a Boardman team with clip on aero bars and averaged 20-22mph for a standard distance last year. I'm very tempted by a tt bike and my options on the scheme would be:

Specialized Shiv A1 (currently slightly too expensive but hoping it drops by January when I get the voucher)
Felt S32
Bianchi Crono
Unfortunately I can't get planet X bikes on my scheme so a Stealth is out of the picture

My question is am I better off sticking with my boardman and getting a bike fit rather than buying an entry level tt bike? I would also have to buy aero accessories and would probably upgrade wheels at some point. What do you all think?