I just got a Wrath, one of the new mid-range wonder markers like the Ion or new BKO. I was expecting to be able to fire at a true 14 BPS with this hopper because I read Warpig's tests that showed this gun could consistently feed 14 balls into a matrix in 1 second. Needless to say, I was greatly impressed, this is better than an Egg or a new-model Apache. So I bought one (unfortunately I had to get an advnace on my allowance!) Finally after working and saving for a month or so I got the money to buy a case and go play. My gun impressed me a lot! I reviewed the Diablo Wrath, so if you want to check out my review on it, you can click on my username. Unfortunately, When I began trying to rip, my hopper just couldn't keep up. Maybe I was expecting too much, but I doubt I was getting much faster than 15 BPS in bursts, and my gun has eyes, so I wasn't too worried about breaking any paint, but it wasn't firing very quickly. And before someone says something stupid, I checked the batteries, and they were fully-charged, and I made sure the thing was turned on as well. For something Warpig claimed to be the best gravity-fed hopper out there, I doubt a great deal that it was feeding like I needed it to. Yes, it has several great plusses, and I love the magnetic door idea, but I just think they're charging too much for a gravity fed hopper that i don't really think can feed at 13 BPS.
If you're using a spyder or something, this hopper would be great. But when you make the switch to mid or high-end, this gun just will not cut it. I recommend highly for spyder users, but it doesn't live up to the 14 BPS hype, so I'm giving it a 7 out of 10, which is a pretty respectable rating in my eyes.
Like yourself, I normally trust the WARPIG tests to be pretty accurate. It is possible that the feed rate was varied by an external factor, such as any blowback your marker may have disrupting the feed. It should certainly be capable of feeding 13bps.
You state that it is the most expensive loader in it's class - I don't know where you bought this, but it is normally one of the cheapest agitated loaders you can buy. The only agitated loaders you generally find cheaper are the always-on types such as the VL Quantum and Ricochet Rhino.
I would expect either the Apache or Evlution 2 (particularly the 2005 models of each loader) to feed faster than the Reloader 2. Sticking with the WARPIG example, the prototype Apache they tested fed 18bps with the same Matrix test rig. They later tested the production model at 16bps, but with increased reliability. The new 2005 model Apache has been upgraded to produce the 18bps feed rate of the prototype, whilst retaining the reliability of the early production models. I don't have the test figures to hand, but the feed rate of the Evlution 2 with Y-board is comparable to that of the Apache. VL have also improved the 2005 model of the Evlution 2, claiming 17-19bps.