While I have no experience with Piob Mohr of Scotland, I would like to add my vote for the regulation doublet. The Sheriffmuir does indeed look quite good when tailored properly, but it is, in my opinion, a bit much for most events. The regulation doublet stands out in a sea of Prince Charlie coatees without making you look like Humphrey the Peacock (and avoids the heat issues that MacMillan of Rathdown mentioned).

If you can't resist the Sheriffmuir, MacCathmhaoil's recommendation of a hunting stock helps to tone it down a bit.

If you do go for a regulation doublet, just make sure the tashes aren't too close to the front opening; I've seen a number of cheaply-made doublets that have this problem which tends to pull the front of the doublet open and cause it to bulge at the sides even when the wearer's arms are down.

Good luck!