Had a chat with a couple of people who know a bit about S2’s and I understand the brakes aren’t that different to the S1 so I stand by my earlier comment that you are not boiling the fluid. To put this into perspective, on Simon S’s race car, which I’m driving this year, we run standard S1 calipers/master cylinder etc with bog standard brake fluid (just Castrol Responce Dot 4 from the local service station) and have no problems at all and we are running endurance races on slicks.
Can I recomend that you go to braided hoses to start with and if there is still a problem to check/change you caliper seals as it’s common to get air in through them.