Where are you located?
I have rollers near to me (AVA beside Glasgow airport, about 5 miles away). Price has gone up, 50 quid a run these days!
De-cat, zorst and filter will get you a wee bit more power, dont expect to be eating supras though with them.
With NA engines, basically to get more power you need to free up the air flow into and out of the engine, when thats done as much as it can, all you can do then is spin the engine faster. Turbos are far easier to get power from.
TBs are not cheap, cams are not what I would class as cheap but they do give an increase and are reasonably easy to fit.
Cams, filter, zorst you shoudl see at LEAST a 10bhp increase and a similar amount in torque.
With decent headwork you should get another 10bhp and you should get a gain everywhere in the rev range if its done porerly.