Let me say this at the outset - don't buy a car because you want the fastest!
Yes a mapped Mk5 GTI is quicker than a standard Focus ST, and to be honest even though the 130 has a bit more grunt, I reckon a mapped Mk5 would be quicker due to the extra torque. But then an ST can also be mapped (a 130i won't really see much as it's naturally aspirated). So if you want the quicker car...
...but Rhyso has also made a very good point. The GTI, although underpowered on paper, is actually a very good drive. It's pretty well balanced and the power is both strong and consistent. The engine does not have an obvious flat spot and in the standard GTI there is minimal turbo lag. It's definitely not the quickest horse in the stable, but the GTI is one of the best drives. There are cars with more power as you have mentioned but few of them will feel as sweet on a good road.
It's also one of the best looking IMO (subjective of course) managing a perfect blend of subtlety, sportiness and class.
Judging from your current car, I don't think what we're saying is entirely alien to you. The Fiesta ST is hardly it's own class leader in terms of power. The Corsa VXR for example has 190bhp and yet I'd imagine a well driven ST 150 could comfortably hang on it's tail down a good road. The reason being that the ST is a pretty balanced package with enough power to be quick but without corrupting either the grip or balance.
But, if power is the ultimate goal you are after, get an Edition 30. Map that and you'll be in RS levels of power.
And let me add in, just a closing thought, the Leon Cupra. It shares much of the GTI's mechanical underpinnings but offers a good bit more power standard (as well as excellent mapping potential) whilst remaining much better looking and more grown up than the ST (although the 130i beats it in maturity).
Verdict:
1. GTI / Cupra
2. ST
3. 130i
