Switching to a taller, high aspect ratio board would be one thing. Taking the existing board and making it a foot longer is another.
The TheMightyHobie18 has a very low aspect daggerboard compared to modern designs. If you just make it longer, I suspect the added drag would pretty much cancel out the increased lift- you might point a little higher, but you'll be going slower.

If you could find a pair of modern boards and modified your trunks to use them, then there would probably be a benefit. There was a discussion about doing just that on the Hobie forum not too long ago.

When it comes down to it, you can make lots of fancy changes to the 'ole 18, but in the end, it's still an 18. Enjoy it for what it is.

sm