I wouldn't be afraid of using a lot more on the exhaust, +12 degrees would be no problem.

I haven't tried 30 degree seats myself, but it is something that has been done in other makes that run in lift limited classes. They work because 30 degrees creates a bigger gap for the same lift which helps low lift flow, but hurts high lift flow. Since the B20E exhaust ports are poor at high lifts, you wouldn't actually be giving anything away. Now that I've put a little more thought in to it, you would have to use inserts to do it, because the original 45 degree seat takes away meat needed for a 30 degree seat. You couldn't use stock E valves either because they have a stellite face, you could try B valves however.

I use to do a little club racing years ago, ie, autocross & hill climbs, but mostly I'm just a hobby guy who spent too much time playing around with B20s & B30s.