Elitist!
So in all seriousness, how does that work- do you shoot a few grades the old-fashioned way, and then compare the results to what the GPS gives you? I would have to think that when it gets that far out of whack, you're back to grade lasers and what have you?
To the OP- I gotta be honest, I shot a property yesterday that drops 16' in elevation from one end to the other, using my $500 laser transit from Home Depot. In less than an hour I located windows, doors, deck, driveway, existing trees, shot grades all across the back on a grid, and photographed the site; it'll take me another hour or less to draw the basemap. I love the gee whiz toys as much as the next guy, but given that the awesome stand-alone systems (Bartlett uses the Ghostbuster-backpack looking setup that starts at $50K per unit) are so expensive, and the cheap ones have a margin of error worse than +/- three feet, I just don't think you're going to find the better mousetrap just yet. Maybe in five years, but not now.
Of course, if you DO find it- by all means, sign me up!