I was in touch with the creator of PPSSPP a few days ago, and while there are some speedups ahead of us in the future versions, it's likely going to be marginal from there on. 
I.e. JIT improvements, maybe 10-15% increase in FPS, max. Neon implementation, maybe a couple of percents, too... 
And any optimization from there on is going to represent a significant amount of work. 
So, what you can expect from there on:
- fixes in compatibility
- slightly better FPS but not hugely faster
- you'll need a Pandora successor to play all PSP games properly and full speed  
