I think the 10wt will serve you well for permit and medium sized tarpon, jacks & barracuda. Actually many tarpon flies are not as big as you'd imagine. Down in the Bahamas, we used orange palmered flies about and inch in length and the tarpon hit them with relish.
Any Belize regulars here offer any thoughts?
Also, a 10wt will subdue a 100lb fish in short order with the right technique - just keep the tip down and put maximum pressure on the tippet. I like to practice busting tippets in the back yard before a trip - amazing how hard it is to break a 20lb tippet with good knots - also its nice to know what 19lb of pressure feels like.
Whilst probably true in most cases, from experience, when targeting permit the longer you can cast with accuracy the greater your chance of success. I have yet to pursuade one to eat my fly but the longer the range the more positive the interest.
As for your issues with the 8wt its hard to tell. I would try different lines. I once had a clear intermediate which claimed to be a 9wt that wouldn't even load a 7wt. The manufacturing process does produce the occaisional "dudd" - maybe a 7wt found its way into the box? Going up a line size might help but I found that whilst it works fine at short ranges, the additional weight can have a dampening effect which will also compromise tip speed and distance. Also a 9wt line is going to offer more wind resistance being somewhat fatter - especailly with a floater. Your constant companion on the flats will be the wind - particularly during the winter months.
Definitely "Try before you buy" with the 10wt. Your dealer should accomodate you. If he won't then find one who will!
Have a great trip and post a report with loads of pics