Surely it's easier/faster to source a STI manifold/ECU and bolt/wire it in rather than swapping heads over etc
Tbh when I fitted an STI engine in to my WRX I'm sure I read somewhere that you can block up the AVCS oil feeds and run it without, Although the performance will suffer (but as you said your not that bothered).
Personally it makes sense to me to do it properly first time round, But I don't know your circumstances.
Seems a shame to lose the advantage of the AVCS.