Can an internally wastegated turbo (i.e. HRC Super 20g) be converted to an externally wastegate setup without compromising performance? Is it as simple as removing the wastegate and welding the wastegate flap closed?

From what I have read never did it myself. Yes that is all you need to do to go external. welding is not easy on the exhaust side so if you do not have the proper skills to do the job have a shop do it for you could not cost to much maybe at the max 100$.

I know it is dsmtuners.com but good information.

http://www.dsmtuners.com/forums/newbie-forum/221019-how-do-external-wastegate-internally-wastegated-16g.html

I didn't see where their would be any problems and didnt hear any problems experienced when I searched online . But I always like to get some opinions/experiences before I try something I'm not 100% sure on.

Did the very same thing for my friend last year, if you look below the pizza you will see an aftermarket TIAL waste gate welded to the stock manifold, then routed to open dump.