Actually I have, and on various 5.1 and 7.1 speaker setups. In order to get the level of clarity and immersion, you'd need a pretty expensive setup. Meanwhile, the Mixamp with a really good open headphone can give you some ridiculous immersion and accuracy. Whereas you rely on audio cues from speakers, the Mixamp envelopes you in a 360 degree sound 'field', that can't honestly be labeled as being 5.1 or 7.1, as the sound is coming from all around you. You can't TELL which speaker is being used, as there's no restriction as to where the sounds are being placed around you. I'm a heavy FPS gamer, and the Mixamp gives me a ridiculous advantage against those who don't use headphones. With a headphone, you're completely immersed in the sound, with little to no interruptions unless you allow it.
Try out the Mixamp with something like the K701 or K702, and tell me just how inaccurate it is. Me, along with quite a few AVSforum regulars have been gaming with a ridiculous amount of headphones and speaker setups and most of us agree that the Mixamp with a good headphone will edge out most discrete speaker setups, until you get to some pretty expensive speaker setups, but then the Mixamp isn't there to compete with $900 systems.
And if you've been gaming on regular stereo (no virtual suround processing) all your life, it DOES take time to adjust to Dolby Headphone, but once you have accustomed yourself to the sound and positional cues, you'll never wanna go back to strict stereo. It doesn't even come close to me.
Again, the item to get is the Mixamp, not the A40s. Get the Mixamp and a pair of AD700s (if you don't mind the lack of bass). The positioning on the AD700s is top notch and a great way to demo how well Dolby Headphone works with FPS gaming. The K701/2 is a lot more expensive than the AD700 but the sound is an improvement in every single way.