The Adeptus Mechanicus do have a few things in the bag they don't let just anyone have. But rogue traders are in that small group they might feel obliged to provide for.

I'm trying to brainstorm what highly unusual feats the Genetors and Magi Biologis could do for someone who has gotten on their good books.

A few notions:

Inquisitor Lord Hector Rex received augmentation "similar genetic augmentation to that of a Space Marine" some time after his psychic abilities were judged on Terra. To the point he was effectively astartes in stature.

http://wh40k.lexicanum.com/wiki/Hector_Rex

Imperial Armour 7

Malgrysian bio-forging from Dark Heresy Radical's Handbook: tech-heresy but useful as a proof of concept. A more expensive and legitimate treatment may exist, adding unnatural toughness and other benefits.

Well the first thing to look at is some of the cyber stuff. A lot of it can be done with less obvious mutually exclusive biological versions instead. Instead of muscles implants, you make the muscles stronger, instead of geting plasteel plates you get flexible chitinous plates. The ability to heavily resist poisons or blood loss. Then there's the various unnaturals altogether. And of course, just straight up steal stuff from the geneator explorator alternate track. Off the wall stuff. Natural weapons, ranged included maybe, regeneration, wings, extra limbs. Beneficial mutations from various gamelines. Even splicing them into psyker.

A lot of these can be really non obvious, a lot of these are really in your face. But once you start splicing them up, you have an excuse to make almost anything available.

Cool notions. I suspect a lot of heretek biological research is really just attempts to relearn what Magi Biologis already know, or to create (relatively) mass-produced versions like the bio-forging.

and just want to link this in from only war

https://community.fantasyflightgames.com/index.php?/topic/131892-gene-enchaced-guardsman/