The location of your mainsheet has no effect on performance.
Moving it to the cabintop will increase the load on the sheet and the boom signicantly. You may need more blocks (pulleys) and/or a stronger boom. You would need to connect the mainsheet to the boom with several new blocks.
Moving the mainsheet will cost you a bundle and you would need to do some careful homework off the board to do it correctly.
Go sailing, and think about the subject again next year.
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