ASG do brand them, yeah.
As for the performance, I've heard really mixed reports, some say they're amazing, others say they're terrible. Personally I wouldn't want to gamble when they cost upwards of £500 if you can find one.
You can get CO2 mags for them though, which might make them a little better, the bad thing is that each mag is going to cost you £100 =/
I'd say if you were to get one, just look at getting it set up with a HPA rig, 'cos then you'd have a pretty formidable rifle.
Do you know there's a spring one though? Why not just get that? £300 from JD airsoft, comes shooting around £400 and it's aparently VSR system compatible. Though I've still not had anyone confirm that 100%