Most automotive certification should/is done on mid limit components- i.e mid limit car on weight regs, power regs etc etc.
Declared figures are also only that- there's nothing to stop Bugatti declaring a Veyron hitting 0-60 in 10 seconds, providing they can certify it, why they'd want to is a different story.
Around the time the G40 was launched VW also had sales of the Golf GTi, Corrado G60 etc to consider, I'd hazard a guess these gave a better contribution the a Polo G40. Arguably not much of an inssue in the UK as sales of the G40 were limited to 500, but in mainland Europe they had unlimited sales.
Real world terms, the G40 is a quick car, its ability to have full boost instantly at any speed can catch a large amount of newer cars off guard- especially as most are turbocharged.
If you're after something which is as quick as possible for £3k, you're looking at the wrong car- get some turbo Jap scrap, or Swedish power, Saab 9000 Aeros are dirt cheap, and even with 300bhp will need a 190bhp+ G40 to see them off (my car has the battle scars to prove).
If you're looking at making a G40 as quick as possible 0-60, mine currently stands at 4.8seconds, that's neither standard, nor cheap!
Pete