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(1) Compression ratio is hugely important too and engines performance, work out your compression ratio and the combustion chamber volume you need, make sure the head you are buying - especially if its of the shelf - has the right volume. David Vizards book 'Tuning the a series engine' has invaluable information in it concerning this.

(2)Valve sizes are no guarantee of flow, just because it has big valves does not mean it flows big, take a valve out and look into the port, the guide should be bullet nosed, and is the seat unnecessarily wide, maybe a wide 'pre cut' to the seat ? Are the seats cut accurately for height, run your fingers across inlet and exhaust differences - are they the same for all the chambers?

(3) Look down into the ports, is there a raised section around the guide (a boss), all our heads have these removed for full flow is the finish consistent across all the ports, and not highly polished - a polished finish can actually reduce power due to fuel being drawn out of the air, due to surface tension.

(4) Hold a suitable gasket up to the manifold and chamber faces, have the chambers and ports been accurately modified ? Usually on cheaply modified heads the ports are flared out to meet the gasket giving the appearance but not performance.

Most mini specialists that buy in heads will base their prices on 50-100% mark up!