I think it would have been around 87, the details used to be on the railUK website but seem to be no more. Basically the French engineers didn't think a 3rd rail system could be operated safely above 100mph, hence a demonstration. This was before the modified 33 was ever thought of.
The main problem with that sort of speeds was impact damage where the conductor shoes came into contact with the ramps, this could of been solved by longer run up ramps but then normal 3rd rail trains had a greater risk of being gapped.
The 4REP was used as they were known to be able to exceed 100mph, this being proved during braking trials to try to up their speed limit to 100mph.