I´m going to retire my Hämmerli SP20 and will replace it with a MG-2 RF pistol. The Hämmerli will stay but shall be used for informal, but classic rapid fire training at 25yds. The idea is to mount an Doecter RDS for that purpose if there is a benefit. Will there be a benefit with a red dot in 10 second strings shot form the 45° ready position? Has anybody experience with such a combination?

OTD I'm not exactly sure I follow your description of the shooting challenge. But let me say that based on my own personal experience there is literally no activity at the distance of 25y that a red dot gun is not better than an iron one. I'm faster and far more accurate with a dot at that range, by a lot. Yes at extremely short ranges I can use irons faster, but that simply is not the case here. As far as I know in every pistol sport with targets at range and where they allow optics the optic people are faster and more accurate. At this point I consider a gun without a red dot as a challenge and deficient. FWIW>

The pistol is held one handed in a 45° ready position and it can be moved into the target either on demand or when the target is facing the shooter. Time is starting when the target is facing the shooter. Then there is 10 seconds for 5 shots. Best hitting zone is a 10 with a diameter of 4in in the center of the target, second best is a 9 with a hitting zone of 8in.

I´m not sure yet what aiming mark I shall use, if I shall use a small dot that is covering the 9 or if I shall take a larger one I can use to reference with the outline of the black area.
The other issue I have is if muzzle flip is distracting the sight picture. In other words would a dot require some fine tuning on the recoil absorber of the pistol because iron sights are might not be as senistive.

OTD. a little less than 2 second splits (if I follow your description) is not super fast. Especially in this caliber. Based on my experience recoil and muzzle flip are simply not an issue in this with respect to aiming and recovering the sight picture. Dot size is always a tradeoff and more difficult to pick an optimum. The sport I do has 8" plates at 25y and its binary, you hit the plate or you don't. I've tried both methods a dot that covers the plate and a smaller dot that you center in the plate. The smaller dot system is overall much better. Yes it takes a bit of extra time to get the dot because its smaller, but you get it all back in the aiming part as its much much faster to center something than to cover it.
FWIW>