Someone let me try their bolster tuned mp5 at my local site a month ago. It's was definitely good, don't get me wrong, but I'm not convinced it's £500-odd good. Trigger response was great, cycled a lot smoother than anything I own, and the range was good airsoft range (60m maybe a hair further - hard to see without a magnified optic but similar to most of my better guns).
I just can't help but suspect you'd get most of the way there with a brushless motor, and 11.1v lipo, and a decent hop rubber. Add to that a perun with precocking and it'll be snappy AF.
Perhaps you need to be a bit more of a connaseur to properly appreciate them, but if that mp5 was on sale I'd be very tempted. It's was pretty cool to shoot.
They did a bolster mk23 a while ago and if you read through the parts list it was a pretty good deal from memory. Not sure on the aegs though but I'd imagine they're adding the parts at a reduced markup to make it more appealing.