No, the only real function of the extra three feet would be to reach the cockpit. It would not pass through any blocks or sheaves, and at most it would only be under load to lift the mainsail the first three feet, then the splice would be in the coil of halyard laying in the bottom of the cockpit as the main was raised the rest of the way.
In all seriousness I could probably just tie a knot to get the extra length, but I was looking for something a little tidier.
How do you plan on routing the longer halyard back to your cockpit? You don't have turning blocks at the base of your mast? No deck organizers? No clutch? No winch to adjust tension?
Do a core to core splice w/ a piece of single braid, bury the cover into the single braid, only go one size smaller than the current diameter.
1. tie a slip knot about 6' from the end of your halyard.
2. pull the core out through the side of the cover about 12" from the end.
3. Milk the cover back cut the existing core back about a foot.
4. put the core from the old halyard into the new halyard about 8-10" from the end of the new halyard.
5. Now put the 8-10" of the new halyard core into the core of the old halyard.
6. lock stitch the cores together.
7. milk the cover over your core-core splice. Use gloves, this will take some 'Jesus' strength.
8. you should still have 12" of cover flapping around. Bury the cover into the new core father than you think it will go, then pull it out of the side of the new core.
9. tapper the cover, then milk the new core over the tappered cover. Lock stitch and admire.
Just a modified core to core with the common practice of stripping sheets and halyards. http://www.neropes.com/SPL_DoubleHitechTaper.aspx