I have used the 393 quite regularly with a Sig 300-800. I find it performs really well with nice fluid movements up and down and side to side. Not the most attractive piece of kit but very well built and good value for money compared to some Gimbal type heads.
One thing I would say however is you may find it over-kill for a 400mm f5.6 its quite a heavy head and for the 400mm you'd be better off with a smaller lighter head IMHO. For the 500mm f4.0 though i'd say it was perfect.
Probably Ill order the lensmaster this afternoon. I think that for static subjects Manf would be better (more stable) but when you want to go for birds in flight and animals who move faster from side to side lensmaster would be better (only have to adjust one knob)
I've been a 393 owner and appreciative user for quite a few years, first with an EF300 F2.8L and now with an EF500 F4L IS. It's good to see some competition emerging, particularly from a UK maker, and at a price well away from the Wimberley / Kirk area.
For best balance, I use my 393 with the two "U"s together (tripod mount below lens) when mounting my 300 F2.8L and 400 F5.6L, and with them separated (tripod mount above lens) when I'm using the 500 F4L. The tripod mounting plates on the lenses have different offsets from the lens/camera axis, and this affects the balance. The sliding and lockable lens mounting plate (with safety pin) is much appreciated.
For stability I've always used them on a Benbo 1 tripod, but I've now bought a Manfrotto 055XPROB, and it will be interesting to see how a modern design compares to a "classic", but very heavy to lug around.