Thanks for the responses so far.
First off, even though I am still learning the ropes of macro photography, I mostly know what I am doing and the diffficulties involved.
So, to address some of the concerns expressed:
1> DF, I am already using the 6x finder with the M screen and using arial focussing.
2> I am aware of stability, focussing and working distance issues. I am focussing using a rail and I use tubes instead of bellows because I find it easier in the field.
3> Claire, I have a couple of cine lenses and use them reversed. I have gone upto 10x with them.
4> Chan, longer lenses aren't necessarily better at above life size because you would need enormous extension to gain magnification. I have the standard barrage of 35mm lenses from a 24mm to a 180mm that I use for most macro work around life size but I can't find an easy way to use a 100mm-ish lens at 5x. And with a reversed lens the working distance is always around the film-to-flange distance so I don't think a longer lens is going to gain me much.
So, even though I could use the 12.5mm cine lens to get 10x, I am hoping that a flat field 28mm would give me very sharp results just like the el-Nikkor. I know that it will be diffraction limited to about 5x life size (after which effective aperture will go beyond f22) and that working distance will be less than 2 inches. Given all these clarifications, I would appreciate any advice/opinions on this lens.