I'm currently testing the QGMLWB layout (I call it QG-B for short, to distinguish it from other members of its family), and my WPM has dropped ~95%, which is a smaller plunge than I was expecting. In time I do think that I'll surpass my QWERTY speed, and even in this early stage I can feel it considerably superior to QWERTY.

Also, I do think QG-B is pretty optimal (if that makes sense). In that regard, I think that Dvorak, Colemak and a couple others are also good solutions. In the end I don't think that there's an absolute and strictly optimal layout, because the solutions to the optimization problem depend on your criteria for 'best layout' (and that's somewhat subjective), not to mention that they're stochastic (random layouts are tested a few thousand times, and the one remaining in the end is considered the 'optimal' solution of that run), because in order to find a truly deterministic solution we'd have to compute ~10^26 layouts for any given set of criteria!

So, QG-B might not be a truly optimal solution, and its criteria may not be THE criteria, but under those criteria it IS better than Dvorak, Coleman and others (not by much), and those are criteria I value, so I went ahead and chose it.

And yes, I did type all of this in QG-B, and it took me like 1 hour! Learning pains....