From most of what I've read over the years about Zetec tuning, a fairly highly tuned Zetec (turbo route excluded) such as Al Pid's, will near-enough match a standard V6 for power and torque. A fairly highly tuned V6 is... negligibly quicker than standard - they just don't respond well to tuning. I'm sure Jamie Duff has said in the past that this is because it was designed and built almost perfectly to brief, so the 'unexploited potential' one can often unlock in engine tuning just isn't there to be had. Yes, you can source and add various ST200 addenda (or even swap in a complete ST200 engine), but you'll be spending hundreds for every BHP or ft/lb gained, with a realistic upper limit of little more than 200 BHP.
Zetecs can obviously be turbo'd. This has been done countless times in wider Ford tuning circles and at least once within this very club. It takes relatively big money to do correctly (ie reliably) though and, as has been said above, will also mean big fuel bills.
V6s can be turbo'd, as evidenced by a quick look at our American cousins over on NECO, but I've never personally heard of it being done over here. Supercharging is possible and has been done once in this club, the result was very powerful (350 BHP) but far from reliable. Both of these options are proper make-you-feel-sick expensive, which goes some way to explaining their rarity.
Bang for buck? I think the most cost effective tuning option for a Cougar is a complete ST220 engine transplant. The Bolton Mafia can do this drive in/drive out for about £1500 and you'll instantly have 220 BHP of solidly reliable 3.0 V6 power and comparable fuel economy to a standard V6, with scope for tuning to around 240 BHP with the same sort of filter/exhaust/map mods that most people chuck at the standard V6.