I did my first tri (olympic distance) about a month ago, on my roadie. Came in slight better than halfway in my age group (M35-39). I would say the majority of the people there were on roadies, with a scattering of MTBs!?!?!? and maybe 1/6th of the people on actual tri bikes.

Get a set of clip on aero bars, and that should be about half the benefit of a tri bike, and see if that position is helpful for you. Maybe a nice aero front wheel (front wheel aero is more important than rear, except you can't really run a disk up front due to crosswinds)

Other than that, get your roadie professionally fitted, if you have not allready done so, and tell the fitter you are looking for good aerodynamics, since you will be racing in non-draft legal events (aka triathlons)