Just had mine done, no problems whatsoever regarding distortion.
I bolted a bar across the bottom and yes, the flat panels did ripple in places but now it is all bolted back together none of it is noticeable. I drilled extra holes in places where they aren't noticable and maybe this helped? I cut the mesh out prior to galvanising and bonded it back in afterwards. Taping the holes doesn't take long but when it came to the nylon inserts I decided to trim the insert rather than filing away the zinc and risk exposing bare metal. Some people suggest filing the holes out first and then letting them fill back up again with zinc, would probably work but you could end up with some sloppy fits? I also took lots of photos and measured where the smaller holes were as they can be tricky to spot when full of zinc.
My bulkhead was new in 2000 and I properly Waxoyled it at the time so when I came to remove it from the vehicle earlier this year the only evidence of corrosion was on the O/S inner-wing bracket.
I had 20-items cleaned & dipped including the bulkhead, battery-box, rock-sliders, door-sills and assorted brackets. Including collection & delivery (I am 50-miles from the galvanisers) the total invoice was £144 inc VAT so not expensive.
I am a real fan of Waxoyl so even though it has been galvanised I still filled it full of the stuff before fitting for no other reason than I could (turn it upside down, tape up all the holes and poor the stuff in). I completed the rebuild this weekend and with the recent heat I am just starting to collect the excess Waxoyl as it drips out.
I didn't have mine painted as by the time the doors, wings, windscreen frame, vent-flaps etc are fitted it doesn't look out of place and it also matches the capping (which I had galvanised a couple of years ago). A nice pair of billet windscreen brackets in silver look nice though, better than the painted ones which I fitted and then immediately removed! Might do the door-hinges as well?
