Good Triathlon fit with 140mm stem, but not necessary good handling

Hi. I have my fit dialed in on my triathlon bike on the trainer and I took it for an outside spin (feels nicer outside). To do it I needed a 140mm stem. People will say that the bike is too small for me, but any size bigger and I cannot straddle the top tube or barely straddle. I have long torso, short inseam relative to the standard frames sizes.
Handling seems fine, except when I am barreling down a hill. Actually handling is fine going down winding descents in the aeros , but it is scary cause my head is so far foward even scarier with wind, I am afraid that I will flip over the front if I touch the brakes or hit a small pebble. My bike is an older 54cm Cannodale Slice (the one style that won a few Kona’s ). I have found frames that are a little longer, by maybe 15 mm at most which will reduce my stem maybe to a 120mm. Is that budge forward worth the change to a new frame. I am looking at a Ceepo Venom. It’s advantage is that it has a bit longer front-to-centre so I will be position 1 cm more in the middle of the frame.

No problem on a flat course.