suppressed AR Pistons - is there still a significant blast from the piston?

Basically, piston systems will vent gas out away from you at the front sight block. This does give a bit more noise, however the tradeoff is that the gas doesn't fly right at your face.

Surefires are generally not the quietest can in town, especially for their price range. I would look at an Ops Inc, Elite Iron, or AAC can.

http://www.advanced-armament.com/default.aspx?pageId=23
http://opsinc.us/category.php@catId=223.html
http://www.eliteiron.net/elite-iron-suppressors-page3.html

If you can, try and get your can through Major Malfunction: http://www.major-malfunction.com/maj_malf_5u_013.htm

He often has the best prices, and always has great service.

A significant portion of the gas that comes out the rear of a suppressed AR-15 comes through the bore itself and out the breach. If you shoot a bunch of suppressed rounds and then eject the chambered one, it will be covered in soot from the bore backpressure, and you can often see gas streaming out the bottom of the magazine-- this is from coming back through the barrel, not the gas tube (as this path is more or less sealed).