Bax,
I have a GP Lil' Toni, I have to concur that the front end is built a bit light for the type of power being used .60 and above.
Things I have noticed:
Firewall: the Fiber Glass sides of the tapered "box" that extends out to the firewall itself, flex and allow the engine to be pushed off center about 1/8" + in any direction. I feel the lay-up in the front is just too light. (the plane needs lead to balance anyway)

The spinner is pretty, but poorly balanced, a 3" Tru-Turn P-51 looks more scale and really helps on the vibration, with the included spinner one needs to bring a bag of cowl screws to the field as you will loose one or two each flight.

The area around the gear block is a bit light too, mine started cracking on the first couple of flights, I added a plywood plate across & between the fillets and tabbed them in with cloth, much better now.

The landing gear is pretty soft too, doesn't take much to "splay" it out.

I am using a new ST .90 Brisson muffler APC 13 X 8 prop, plane is pretty fast, can pull through huge loops. Lands very nicely.
For the guys with roll problems, check your aileron differential or something as I have a lot dihedral on mine ( I just pulled the wing roots up tight to the built in angle) Mine rolls quite nicely, not quite pattern quality, but great 4 point rolls.