How much more do you think the RM 850 will cost than the Elan 320? If they are close I would go with the RM. Otherwise, I will continue to lust after the Elan. I'm one of those small-to-medium sized guys you referred to (1.74m), so will only bump my head slightly in the head.
I was going to say that the Elan 320 was not necessarily slower since it has a bigger boat (9.25 to 8.90m) but then I looked at the LWL and quite incredibly the RM has a bigger waterline ( 8.81m to 8.71m). It seems that slightly inverted bow is not there only for the looks
The RM is lighter ( 3200kg to 3690kg), beamier (3.42m to 3.22) and it carries upwind 1.5m2 less sail area than the Elan 320. I bet that downwind it can carry as much sail and probably more.
The hulls are very different being the one of the RM based on Open boats and the one from the Elan more traditional and adapted to IRC racing. The beam all pulled back on the Elan can be deceiving but in fact it is not particularly beamy, for instance an A31 has 3.23m of beam, almost the same as the Elan.
The keels are similar but on the more sportive version the Elan has 2.15m draft while the RM has only 1.90m, an advantage in what regards saving weight in ballast for the Elan. The Elan has a B/D ratio of 0.243, the RM has 0.302. The difference in draft is not even near to make for that difference in B/D ratio. So the RM will have not only a much bigger hull form stability as it will have more RM coming from the keel with the boat heeled, and that means a much stiffer and powerful boat....and the performance is there to show it.
Probably there will be some conditions and points of sail where the Elan 320 is faster but not downwind or on a beam reach...and regarding the other conditions and points of sail, the difference is not going to be much, if any.
The Elan will perform better in IRC being easier to sail to its handicap and on a transat race the RM will for sure be faster.
Regarding prices I can give you prices at the factory but they are not meaningful to a boat deliver in US.
The Elan 320 costs 77 990 euros, but you would have to join the performance pack. You can see the price of that and other extras here:
The RM 890 costs 80 936 euros but probably you will not need all the extras that you need on the Elan to make it a faster boat. I don't believe the monokeel is more expensive than the twin keel and unless you want a carbon mast, it is all there, except electronic and other comfort type extras.
NOUVEAU RM890 - Blog RM Yachts | Chantier Fora Marine
Regarding the RM they don't have dealers on the US. You would have to pick up one on the French Martinique. This is the dealer:
RM 890: RM Yachts RM 890 Sailing Monohull on Boats-Caribbean.com
And an interested one since it was the only place I could find more pictures of that test by Voile magazine, these ones: