Motors,
Have not done that. I did try to shift a vibration frequency problem on an already flown and tested Cardinal with an OS .46LA by laminating .040" carbon plate to the outboard (engine mount) side from the spinner to the main spar. It was not an effective mod, and as a add-on of last resort it was also weight that the plane did not need. The nose was already very stiff for a profile, having used a fairly stiff balsa core, and the doublers were real 5-layer birch plywood. The .46 was never happy on that plane. The .40LA actually ran better on it.
The promise of carbon is always that the structure could be lighter if designed from scratch and stressed properly. Mostly what you see though, are "black aluminum" designs that are often heavier. The more stress points or points where loads are introduced, the more likely carbon is to disappoint. Check your design carefully for out of plane loading, point loading and delamination points. You will need hardwood plugs between the carbon wherever those load points are. The usual stuff.
With no plywood at all, I think I would try the .040 since it would be a bit less prone to delam damage from point loads. But if you already have the .025 and have the time to try it.....
The Divot
PS--there is actually one mod that makes the use of light-ply an almost forgivable technique: use post inserts for the engine to sit on. Grind them off slightly above flush so the engine never touches the wood area. Thread the inserts into the engine bearers. It's a common technique used in F2C planes that works.
I