The standard ones vary a fair bit so gains vary according to that, also according to what other mods you have, if you have a set of hot cams then the standard inlet becomes more of an issue than it is on the standard cams.
Its not the sort of difference you are going to fit it and drive down the road and feel like the car is transformed though even in the best case TBH
As mentioned already, Neil Roper is the best person to go to IMHO, although if you go via Mike at MWM he normally keeps a set in that he can send out prior to your exchange, where as Neil will want yours and might keep you waiting a fair while as he does a lot of big contract work on race cars etc which is likely to get priority over your one set of inlets sadly, so Mike tends to get a few sets done at a time.