I have been reading through the forum for a few days now as I have a 1999 1.6 16v Astra G. Trouble free for the last 4 years. After reading the technical articles I derestricted the air box. Tonight I picked up the Vectra C intake and adapter from Lookers and fitted them. (£13 compared to the silly amount Regal wanted)
The difference is very noticable compared to the paper filter and standard air intake/box. It was quite a job and at one point I was considering getting the local vet to help! The car isn't any faster, it is just more responsive.
Next on the list will be a front strut brace and possibly the rear one, then a change of shock absorbers.
It's good to find a forum with people that know what they're talking about and the "how to" guides are great.