I believe OTP performance figures you quote are only based on the departure time (off-blocks) from the UK airport and not related to the time the aircraft leaves downroute, or returns back to the UK.
The only impact the longer planned flight times would have is create more 'buffer' for turnarounds back in the UK should the flight get back early. This might ensure any maintenance etc.. can be completed before the aircraft goes out again.
esvdx