No official figure for M35as I don't think they had an OE towbar kit.
As much as it pains me to say this (isn't much I would swap my M35 for), I think you may be better off going with a different car.
If you are towing above 1500kg for long periods of time, I would be concerned about the allow subframe.
If you need Nissan, try a Murano. Then get a 350z Twin turbo kit for it
Yes the M35 responds well to mods.
I do not know the dimensions of your GTR FMIC, but I dare say you could make it work. (What are the dimensions).
In the "Aftermarket M35 Exhausts" thread, you WILL find a dB rating. I made sure to include it.
I didn't include a performance increase rating as there are far too many variables and I just don't have the money to buy them all and test them out!
Performance wise, what is your aim?