I found a TDo5 16G laggy as on my WRX, full 17psi of boost at 3800rpm. How does a little 1.3L even spool it?
This ^^^.
AFAIK a TD05 16 G, would be a "seriously laggy burden on a 1.3?? And yeah 300 hp on the top end sounds like a lot of "Fun" in a light weight car buttt, you don't drive a car on a "Dyno??" You drive in the real world and in the real work ... I tend to think it would suck?? Most likely it won't produce any boot until ... 4000 rpm?? It would suck in traffic and day in and day out it would tend to get old fast, I would imagine??
A better option in my opinion would be to keep what you have (turbo) and pursue this as step one.:
http://www.ardideas.com/TD04-19T-Conversion-Kit-Subaru-_p_2211.html
That is a "bolt on option" a larger hot side to flow more air and the same cold side ... so spool up "won't change!" And if you want "more" ... then a Stand Alone with "proper tuning" that would allow you to run more "ignition timing" on and off Boost, would give you more bang for the buck then slapping a bigger turbo on a tiny motor would?? Just a thought.