There are a lot of thermostatic showers around but most seem a bit shy of giving their flow rate (and non-thermostatic ones are also as bad). The only info I could find was from Aqualisa at the end of their Opto Thermo brochure where there is a comparison of 10 different makes. Naturally the Opto Thermo comes out on top but even so it still has an impressive flow rate of ~10l/min.