'Highflowed' is a common term used here in Australia- one that I wasn't accustomed to when I moved here- and everytime I asked someone who was selling a 'highflowed' turbo what was actually done to it- no one ever knew. Its a bit of a 'load of shit a.k.a wankfactor' if you ask me- be careful what you are buying.
If you are going to spend big bickies modifying a turbo, then why wouldnt you put the exact specs of it- instead of just saying 'its been highflowed'? Also- if it was really that good- why would someone be selling it? Someone could clip a turbo and say its 'highflowed' and the average joe wouldnt even know!
As for the 2nd part of your question- stick to the TD series turbos mate- big16g FTW.