Turbo for '99 V70XC; which one 13G or 13T or upgrade to 15G or 16T?

I need to replace the turbo on my '99 V70XC AWD automatic with the 2.4 l engine (code B5254T). Can anyone tell me what the difference between the 13G and 13T are? I am getting conflicting answers as to which turbo is correct--the Mitsubishi TD04HL-13G or the 13T? According to one dealer the Volvo OEM part code is 8601692 which should be the 13T but I've found the same part code with a 13G. What's the difference? And can I upgrade only the turbo to a more higher performance unit without having to do any modifications? Does anyone know a really good rebuilder? Also, I'm a newbie when it comes to the pros and cons of the low pressure turbos vs the high pressure turbos. My V70XC seems to be down on power--or at least what I am expecting from a turbo-powered engine. I never feel any boost or experience any turbo lag, but the dealer "supposedly" looked at it when I had the recall replacement of the throttle body done and pronounced it healthy. Is this because the turbo is a low pressure unit? What settings in the ECU mod should I pay attention to? I also have noticed more white smoke from the exhaust than usual but not clouds of it--usually upon idling and acceleration. Any suggestions here? THANKS VERY MUCH!!