I just got a set of the rear aperture inserts from Tech-sights, and will be trying them out soon. They reduce the size of the peep-hole, which creates a sort of pin-hole camera effect and makes it easier to get a focus with aging eyes.
I tried an initial experiment with reducing the size by putting a bit of masking tape over the regular rear sight and poking a hole through smaller than the stock aperture. It seemed to help - at any rate I got tighter groups - so I splurged the $15.
I'll post a report after some range testing.