Order a set of ported/polished/ceramic coated stock headers with a high flow crosspipe from GimmSpeed and send your stock headers back to them for a core charge refund. Minimal down time, easy install, faster car, retain signature Subaru boxer rumble, prettier engine bay... problem solved and then some
If it is good build up, just a quick wire brush should do. No point trying to get it all off. If you're engine cleaning, look into a product by P21S called Total Auto Wash. I can't recommend it enough.
"We live for a good time, not a long time."
RIP Colin McRae