Thanks for the replies. I am booking lots of different high-end hotels for short stays across France/Spain, and so far I have booked about half with FHR and about half with a (randomly assigned) Virtuoso agent. So when we get back, we'll have some non-scientific data to share on upgrade percentages.

We're going on the tail end of high season (early September) so trying to keep expectations pretty low.

We have a few more to reserve still, so I will look into possibly upgrading to a more proactive Virtuoso agent for the remainder, or check out the Ritz-Carlton STARS thing.

I find a lot of the properties we're interested in in this area are "Relaix and Chateau" properties. Are there any specific tips on who those guys respect the most in terms of booking agents? It seems some of them are FHR and MOST of them are Virtuoso, so so far Virtuoso has gotten most of those bookings for our trip.