Well to help those heads breathe and work well with that cam I hope you have an aftermarket intake.
Also read this website as there is plenty of good info there.
Also in regards to "cleaning up" the heads....well to port them it will cost you some cash and it is usually not worth it cause it will put you into used aluminum head territory. Now the GT40's are a nice budget upgrade and should give ya a boost in power tho.
Scorpion makes a nice roller rocker as well as the usual suspects. Stay away from Pro Comp rockers seen 2 different sets fail on 2 different cars.
2016 VW GTI 2.0 Turbo
1989 Sonic Blue LX Coupe, 349 ci, TW heads, TFS Track Heat Intake, 75mm Accufab TB, 80mm ProM, 30 lb. inj, TKO, 3.73, 31 spline w/ TruTrac, MAC 1-5/8" LT headers w/ H pipe, Flowmaster, electric fan Moates QH
391 rwhp & 424 rwtq...Mustang Dyno 11.5@122mph