The MT 12 is a very strong and reliable motor with pretty good power but it is 2 generations old so it is not as fast as the NS12's. The main area that the NS will show it up is in the top end. The MT12 is easy to tune and very user frinedly so I think it was a good purchase and later on if u decide u want to race or get more speed then get another engine.
With your friends CX12 (I assume), the MT12 should blow it away.
On the max speed that all depends on gearing, size of tyres and other factors.
For fuel i'm not sure what brands are available to you, sometimes that will determine what you run. For Nitro content I always feel 16% Nitro is a good place to start, the higher the Nitro % the faster your engine will be but the life of the engine will be shortened. If you go 20-30% you will also probably need to re-shim the engine too.
Hope this helps