I don't have personal experience with it, however my porting guy, Keith Johnson, has flowed a bunch of Mick's heads. The low rpm flow numbers are pretty mediocre - leaving the bike less streetable.
Its not a "drop in" upgrade either. Valve to piston clearance will be a problem with anything but the stock cam.
My advice: a good 3 angle valve job and stock size Kibblewhite Black Diamond valves. Its worth a few cfm at low lift and will give you a few more ponies than the stock setup without the hassles of an uber-custom rig. (trust me - I know from personal experience)
I have a big-valve head on my 850 cafe racer. Can't say who did the head work as it came on a basket case I bought many years ago. The porting of the head looks good to me, but I'm no flow expert. The head was milled for compression down to the very edge of the valve seat. I took the old pistons and new pistons down to my local hot-rod machinist and had the new ones flycut to match. I checked the the valve to piston clearance with clay using a new headgasket and rotating the engine through a couple of revolutions. After several years running, I have had no problems. Rockers are lightened, S&W springs, titanium retainers, etc.
Like any performance modification, one change needs another to fully realize it's potential. This bike runs dual 34 mm Mikuni's with K&N individual filters, a 2S (Combat) cam, and 1 5/8 diameter Bub Conti replica mufflers. It does lose a little at the bottom end, but I'm also running a 21 tooth countershaft sprocket. Once you hit 2500 rpm she really starts to move. It's a street bike, so I keep it under 7000, but it gets there in hurry. It's fun!