IF you buy this months Auto Italia magazine it has a comparison test between the GP T & M Jets. 120bhp and 130bhp models.
There was only something like 6mpg between them, it was something like 44.5 & 49.8mpg but don't have mag at hand, so little in it.
0-60 times T-Jet 8.9secs and M-Jet 9.5secs IIRC again little in it.
Top speeds both illegal unless off public roads, were something like 120 and 124mph, so yet again little in it.
But the biggest factor I thought was this;
T-Jet kerb weight 1060kgs
M-Jet kerb weight 1205kgs
And as pointed out by Auto Italia this weight is all out front so the M-Jet is not as dynamic as T-Jet in chassis department.
The differance is like having two 11&1/2 stone people sat on the bonnet.
This not only means its going to understeer a lot more, but front end tyre wear will be much higher as not only is it carrying more weight, but the same tyres are trying to transmit more torque to the road, a consideration when it comes to running costs.
So may be worth picking up the mag to get the full report see what they say.