I would stay with the Dot 5.1 as you say it is the dot 5 that is the problem one, change your pads to Bendix or an equivilent race pad, better still get some P4 calipers & a Brembo radial master cylinder, thet set up will not fade.
The biggest, i.e. most effective, change you can make to your brake system is to fit 18x20 radial master cylinder and some good pads. Bendix, the new Brembo RS, Performance Friction, are all good choices.