Having thought about it further, the stone on the golf course is too far to the south-west of Comrie Castle for Coles to have described it as a mile and a furlong to the south. So I now think the original site was in the fields of Inchadney Park, or on the piece of land known as Eilean nan Ubh and Eilean Mor. Along the edges of these fields are piles of field clearance, including stones big enough to be the ones shown in Coles' report. I'll go back again later this summer and make an extensive search of the undergrowth!
I tried again to locate this circle today, and once again failed - perhaps! There's a stone standing in the centre of Kenmore golf course which looks suspiciously like one of the stones drawn by Fred Coles in 1910, and it's in the right general area to fit his description. The golfers I spoke to didn't know anything about the stone, and the clubhouse was shut, so I'm planning to write to the owners and ask if the stone has always been there, and if anyone knows anything about it.
I tried (and failed!) to find these stones today. My mistake was to only read the CANMORE entry, and not download the PSAS report by Fred Coles. CANMORE only quotes a footnote by Coles in which he describes information given to him about four "'great stones' close to Comrie Bridge" - he dismisses these as just boulders. But it wasn't these that he was describing as a circle of 5 stones - the circle is apparently a mile and a furlong south of Comrie Castle (which is next to Comrie Bridge). So I was looking in the wrong area today (as presumably were the RCAHMS, given the bit they quote), but I shall return with the PSAS description soon and see what I can find.