Do you have a 1.4 k series?
The early 1.4s had a 56mm TB as standard, however it would not open fully.
The later engines had a 48mm TB (even the vvc) and the vvc160 benefitted from a 52mm TB.
On a standardish 1.4 you would see no real benefit from modifying the 56mm to open fully, nor would you from fitting a 56mm or 52mm to a 1.4 with a 48mm as standard (maybe 1-2bhp)
I had a 1.8 solid cam running 155bhp with a 56mm TB modified to open fully and it worked well (even if the throttle was a little jerky)
Some people believe the 56mm is overkill and actually causes the inlet air to loose velocity and can make the engine bog down in lower revs, i never found this was the case, then again my engine was fairly heavily modified so on a 1.4 or 1.6 it may well be the case.
52mm TBs usually go for between £90-120 and 56mm go for around £20-30
Beware if you buy a 56mm tb there are two different types (one from the t series 2.0 turbo and one from the 1.4 k seires) they have different sensors on them
HTH :blink1: