Of course putting in a 1.8 20v T with 225 bhp standard is a much bigger gain then you'll ever get from a standard 1.3 or 1.4 or even 1.6 16v GTI, but that's a lot more work and costs a whole lot more money (plus it's risky with your insurance)
The easiest gain in BHP is to just put in a 1.6 16v GTI engine, that's a double of BHP compared to a 1.4 8v. To get that kind of a gain with moddin' your 1.4 8v is very expensive.
But it's cheaper to just put in a (for instance) 270 degree duration cam, put on a 4-2-1 exhaust manifold, a good induction kit and install a fuel-pressure regulator. Buy all this stuff on Ebay chepaly, and you'll laugh your arse off when you see your total price-tag compared to buying and putting in a 1.6 16v GTI block. I know a gut here in HOlland who has just put in a 1.4 8v, modded it by himself with some second-handed parts and is now putting out over 100 BHP (he also put on motorbike-carburators, which is a good gain in performance, it's a lot like multiple-throttlebodies) and it didn't cost that much.
About the Calibra, first of all, Vauxhall suck
(just kiddin') but Calibra's are as heavy as f*ck, so if you gain 10 BHP, that 10 bHP still has to drag that car weighing over 1000kg across the road, gaining 10 BHP in a 850kg Polo, will be more fun then in that Calibra, it's all in the BHP-tper-kilogram ratio's boys
Don't stair yourself blind on BHP, take a Toyota Landcruiser for instance (my dad drives one) the thing has 170 BHP, which is almost as much as a Golf mk3 VR6, but a VR6 is much faster, both on acceleration and top-speed, why? The bloody Landcruiser is a g*dd*mn heavy-weight, while theGolf mk3 VR6 is a lot "lighter", and therefor a lot quicker.
And to answer hardcore's question, I can't give you any numbers of gains in BHP's from a cam and manifold, it's all dependent of the state of your engine, and of course what brands of stuff you buy, or what duration cam, what valvelift of the cam etc. etc.
But I can promiss you, gaining 10% in performance in a mk4 Polo is a lot better to feel and enjoy and a whole lot easier (and cheaper) to gain then a 10% gain in performance in a Calibra
So why did I spend around 600 pound on my 1.3 engine? Just to get it that bit faster (it won't ever go faster then around 200km/h, since my gearbox will then just make the engine go into the red revs) and accelerate much faster, at a cheap price (cheaper then buying anonther engine (for instance 1.4 16v 101 bhp) which I don't know the state of) and it's a great learning experience, which is a major reason for me. Now I can almost take apart my entire engine, put it back together and it still runs
