Jarendt, that's too broad of a question to answer. You first need to decide what you want to do with the car, ie: daily driver, weekend toy only, race track only, daily driver with occasional track time. Once you find that out, contact the custom grinder of you choice (and believe me, everyone has an opinion on who is the best) and tell him what you've got and what you want to do. He'll spec you out a cam that's bigger than the 306 and give you more hp, which is what you're looking for.
To say "what kind of numbers am I looking at with going custom"...can't really be answered. That all depends on what size custom grind you go with. The cam grinder you choose will be able to tell you once you both have decided on the cam.
G/L