And what about horsepower upgrades?
You can get a bit more at high revs by opening up the exhaust, at the expense of torque at low revs. I know, because I had a replacement stainless steel system fitted (it was cheaper than mild steel !). The low end torque suffered a hit, and I hardly ever use the high end power.
Similar resuts can be had by fitting a reground camshaft.
The other way is a custom turbo installation. That should not hit the low end torque, but will use more fuel if you use the extra power.
There are a few threads about that on this board - do a search.
The best way, in general, is a larger engine. Just don't expect to save fuel.