Have to say I don't agree with you workhard, sorry.
At my school heads of department would only move a pupil at the end of year 10, ie halfway through a GCSE course, if there is a good reason, and as many said to Minesh on other threads, in response to the same question, if sets are parallel and doing exactly the same work, with the same predicted grade for a child, not many would see that as a good reason. Personally I think the Chair of Governors would be reluctant to wade in over this. I can't see any reason for a school to give in to a request for other students' marks when the setting has no practical implications for the child.
Aliportico: my DD did Greek as a twilight subject, in addition to her other GCSEs. There is no doubt it was a lot of extra hard work, but she feels it was definitely worth it. There was no option to drop another subject instead (but we knew that from the outset) so I can't advise on the likelihood. Again, I would always advise talking first of all rather than going heavy on the governors or sending letters in - in my view this ought to be a last resort rather than your initial approach. I am sure the school would be willing to discuss any concerns you have over workload - it is a good start that they are prepared to facilitate the extra subject and I am sure they will be keen to put your mind at rest. Good luck.