I went from doing Merckx style tt’s on an old aluminum house brand road frame from Nashbar size 57cm with a 110mm stem slammed to converting that same bike to a franken TT (the old non carbon fast forward seat post from ebay, round tube TT base bar and s bend extensions on Craigslist, 130mm -25 degree stem slammed)
I went from a Merckx TT 20k of around 31:30 to a franken TT 20k of around 29:30 for around the same power.
I then picked up a 2014 BMC Timemachine TM02 ML (standard stem front end) with Chinese carbon aero looking base bar, ski bends, -7 110mm stem slammed. Dropped my 20k down to 27:38 (about 20 more watts than previous)
Before I bought the BMC I had ordered a 150mm -35 degree Trials (the competetive version of road bike party) stem that I was going to use to get lower and longer.
Long story short my limiters were length and height. I needed longer and lower than my road frame would allow but it was still markedly faster than a pure road bike. If you are running a non slammed road bike with a shorter stem you may do better than I did.
Also where are you located? I have the stems and seat posts from my old fraken TT lying around. I am in the San Diego area.