I recently got my first GAP pre73 and the build quality is pretty damn good.
The metalwork is excellent and cerainly no sign of the flaking paint moaned about on here early on in the 73`s existence.
I soak tested mine for about three or four days before letting it do any actual audio work and it has just settled in and done the job.
Re changing the caps and the input transformers, which appear to be the only mods that are universally regarded as worthwhile, the manufacturers of Neve`s original transformers said to me that they could not hear enough of a difference to make the extra expense and voiding the warranty worthwhile. And bear in mind they are THE source of these transformers still here in the UK.
In view of that and the supposedly marginal improvements achieved by tantalising the caps, to coin a phrase, I decided to leave mine alone until such time as I stop liking it.
Dont know anything at all about the HJ kit ones, I`m afraid.