just had a quick blow on the Runyon Custom alto mouthpiece with and without spoiler - the spoiler seems to make things a little bit brighter and slightly thinner,, but I prefer it without the spoiler, it seems a bit fatter and blows a wee bit easier in the low end. It's a fairly modern, focused sounding mouthpiece, kinda jazz fusion-y, but not as thin and bright as a Dukoff and plays well from top to bottom. The phrase 'Yamaha 6C on steroids' comes to mind...for a relatively cheap plastic mouthpiece it's pretty well made with a decent facing curve. The altissimo is reasonable.
The only weak point is the cheap plastic ligature that comes with it which isn't particularly amazing, but Rovner make ones that fit well
In general, I quite enjoyed playing Runyon mouthpieces and only changed to the Lawton 'cos I needed to make myself heard over a particularly obstreperous drummer..
The spoiler itself will only fit certain mouthpieces since it seems to wedge itself between the side rails, so won't fit many other mouthpieces unless the distance between the side rails is the same as the Runyons. It's about 11.6 mm at it's narrowest end and about 12.5 mm at it's widest, so it's designed to wedge in place at a particular point on the baffle
hope this helps