Trying to the thread on an even keel, here's some quick results today. These were shot at 20yds, no allowance made for wind, though you can see that both groups are tight around the same point to the left of target. The left group shot standing normal position, right seated 'alternative' standing.
Some notes firstly the standing group is slightly elevated but this would be compensated if it was my normal position, so I'll disregard that. On the left two shots are highlighted, i started the group without using a sling, so I'm ignoring them. Thereafter we have 10 shots in the left group, 5 shots right.
So to analyze, the standing position produced 10 shots within a 15mm group, with 8 shots in a 9mm group.
The seated position produced one group 8mm centre to centre.
Assuming I'd corrected for wind as both sets show identical wind deflection we can see clearly that these groupings would both have produced 100% hits on a 20 yd mini kill, and if extended to 40yds, then although more ragged to the left, both groups would comfortably hit a 40mm kill plate.
My thoughts, firstly I'm in considerable discomfort now and wondering why I tried this. The standers were extremely unpleasant and I couldn't sustain a full competition course, having shot these. There are only 5 shots on the seaters which I tried later as it got too painful and I'd established a result.
So taking into account that I have the injuries to my upper body I would expect the left, standers, to improve if I didn't have the gentle grind of bones to contend with,I'd also need a bit more practice which, as Chris alluded to, would tighten the group. If I lost the flyers, which came at the start of the shooting then I reckon a 9-10mm group is achievable.
In conclusion, for me, I think the stance would be the lowest factor in the real world when it comes to hitting standers, wind estimation, poor rangefinding/tricky light and competition nerves would be of far greater influence.