I could be way off on this one, I'm only responding cause no one else has so I want to help. Figure out what your ultimate goal is for boost...you say 15, is that you final goal? What size turbo? I'm not sure how much an internal can handle reliably, but I do know that once you get higher with larger turbos, it will be a lot safer and easier for an external wastegate to be able to control the boost without creep or spikes, and a lot of the bigger turbos don't have internal ones for that reason. The smaller gates (internal) don't have as big of an opening and therefore won't be able to control the higher boost. If your looking to go bigger, more boost, I would go with an external setup. I'm upgrading my Deltagate now cause I'm shooting a lot higher, so my deltagate will be for sale this next week if your interested for pretty cheap. I've been running 16 without a problem on it for a while now.

If anyone else has anything different to add, go right ahead.