After some regular and aggressive driving over the past 300km I have to say that the Metric Mechanic modified final drive is great!
Enough has been said about the gearing... the car pulls like a rocket in gears 1-4 and cruises nicely in 5th (not too high rpm).
If anyone is considering doing a 3.46 upgrade you can get gearing only done by Jim Rowe of metric mechanic for $1445 + shipping. This is an alternative to purchasing a 2nd complete final drive. The total prices are close and you can't go wrong either way. Of course, if you want to have the 3.15 as a "backup" then purchasing a 2nd assembly is the only option. However I understand they are not exactly plentiful in supply, plus here in Canada they list for CDN$4000.
Jim states that their failure rate of re/re final drive assemblies is 0% (i.e., no failures).
Realistically any good mechanic with access to a machine shop or machinist can swap gearing in a final drive, but the main problem is sourcing the parts. It's a hunt for a good set of matched r/p gears and it's almost guaranteed that some machining of bearings or shims will be required to get proper fitment... I've been through this twice now (Quaife in E39 540 and now the M Coupe gears).
Anyway, just wanted to give a thumbs up to Metricmechanic.com.