A base extender makes the combination mast + extender softer only because you can use a smaller mast.
For example with a luff of 522, you can use a 490 mast + 32 extension or a 520 mast with 2 extension.
Because a 490 mast is softer than a 520 mast, you obtain a softer combination with the 490 + 32 compared to the 520 + 2.
In your case with the 550 mast, you are already on the longest mast available on the market, so you can't make it harder anymore, only softer by means of a smaller mast and a larger extension.
Large extensions have serious impact on the bending curve. They will always make a mast more flextop.
A top extension will probably do what you want, i.e. making the mast softer compared to a base extension, but again it will also change the bend curve. Hard to tell in which way. It most certainly will result in a complete different sail setup. Perhaps a setup for which your sail was not designed.
Take also a look here: http://www.peterman.dk/windsurf-NP-X...ions-780gb.htm
My take on this, stay with the recommended mast + extension for your sail.