You mention that you can also feel the vibration in the steering wheel. do you feel it in the seat of your britches?? It sounds like your engine mounts are worn out and the engine is sagging onto the chassis causing an abnormal amount of vibrations. I would definetly have a look at them to see if they are still full of fluid, if they are they will be balloon like when touched from the bottom.This is not uncommon.
As far as body flex (remember it is a unit-body with front and rear subframes attached to it) this has been the scourge of roadsters, spyders and convertibles alike since the beginning of topless travel. Your Maserati is no different. What these cars really need is subframe connectors and an X or cross brace,a front "strut tower" brace from coilover to coilover and a genuine roll bar would help as well. Maybe the market is asking for the creation of some bracing.
Check your mounts and put a wrench on all the suspension points and front and rear subframe mounts, this will cover any unwanted component movement.
Lots of luck, let us know how things work out.