You need more off a package (obviously £££ pending) For example :
Exhaust manifold - yes you can buy ebay/tat etc but a "properly made" ex mani will give you plenty of gains....
BUT : depending on your cams - lift/duration etc. A ex manifold needs to built properly ie. tuned length. All about the maths (cant be bothered to google the equations)
Larger throttle body may help but depends what you aim/budget is.
A K+N may make you think your car is faster but it will lean out high up rev range. Therefore you will want it mapped (piggyback ECU or mapped chip NOT a superchip).
Porting and polishing is max power buzz words, if you mean proper porting and valve work ie. 3 or 5 angle valve work (although this depends on the rest of the "package")
I may not be making much sense , but think of this way, you want to rennovate a run down house you don't just whack new paint on the walls and get double glazing if the electrics are fuggered and there is damp and subsidence etc etc.
To summarise, skim the head, get verniers, dial in some advance, decent manifold/exhaust(decat too) and your induction kit will see some alright gains if mapped and checked on a dyno.