Steve, mate you'll be right like Piros said, no worries.
I added a twin box to my Atlantis vex (I over engineered it to the max) and ride the h4 myself. I'm 93 kegs on a 8' board and the vex paddles 100% better with the foil on than without it.
If anything mate you'll get up easier being lighter.
My best advice is to make sure all bolts are as tight as you can make them. My mast bolts into the Pedestal loosened off during my second session where it all started to wobble. Didn't think much of it and caught a couple more and then came in and saw the top inch of the mast bent over throwing an angle into the foil. Took lots of persuasion to straighten it back out and now I double check all bolts and had no more dramas.
Also make sure your mast is leaning forward in the "positive" and not leaning back.
Good luck mate.