I'll reply to this post from the other thread in here as its more appropriate:
1) You can have it re-mapped at Autodelta or Red Dot, replace the air intake (I have with the Autodelta one), new exhaust (more for the looks and sound than extra power but I'm sure it will help) and pre-cats as G5een mentioned
2) Re-map is around £250, the air intake is £120 and the exhaust backbox (Raggazzon, Autodelta or Supersprint) around £300. The pre-cats are also £300 ish
3) With all that done you should be getting about 190BHP I reckon. I had it alldone to my 147 2.0 Twinnie and I think that was about 175BHP
4) Shouldn't affect mpg too much. Didn't on my 147
5) Not sure about insurance. Never mentioned it to them with my 147 but would with my GT I reckon. Hopefully shouldn't be much
Firstly, its difficult to 'add up' bhp gained from different individual mods. If you pick the modifications right, they will all complement each other and the sum will be greater than the individual parts, if you get me.
But I struggle to see a JTS making 190bhp from intake, exhaust and remap alone. Thats over 25% increase (assuming it makes 150bhp standard which is probably more like it). I'd say a 10-15% increase is more like it from those mods, so afterwards probably somewhere between 165bhp and 172bhp..
Something like the GSR intake pipe would work wonders on the GT, as its the convoluted pipework more than the filter which is causing a restriction. Then the exhaust, you could possibly just bash the pre-cats out of the standard manifold for a decent gain top-end. Get rid of the main cat too.
But if you really want more power, change the cams. That'll really liven her up. I think Colombo & Barini do a lot of different cams for the Alfa 4-pot engines.. Worth a look I'd say