You can certainly buy a 90-100 liter iSonic, and get a couple of good slalom race sails to power the board.
Who knows why your friends want to keep you on other boards? I guess you need to ask them.
But, being able to jibe, and your ability to go fast (compared to whom or what other boards, rigs and sailors?) does not necessarily mean that an iSonic and a couple of good slalom race sails will make you into a sucessful racer.
Have you ever raced (in an orgainzed slalom event, not just against your buddies at the local club)?
Also, what are the conditions you sail in?
Is an iSonic really the most suitable board for your conditions?
I know, lots of questions and no answers here, but the only way to find the real answers is to buy the iSonic and slalom race sails (with the high carbon masts they were designed on) and go out and see how you do "on the water".
Sailors do progress much more quickly on the modern gear, so there is a strong likelyhood that you are "ready" for something like an isonic, but unless you are sailing near the front of the fleet in organized slalom races, there is no way of telling if you are
ready for slalom racing.
You have to enter the races, and see you well you do, then decide if the iSonic/rigs was
really the best way to go.
And, (here's the hard part), if you do not do well in the organized slalom races, with gear that is known to be at the front of the fleet much of the time, you need to work on improving your skills (sailing consistently fast, making all your jibes, starting well, tuning your gear for changing conditions).
When you are consistently at the front of the fleet in at least regoinal level racing..... then you are fast!
Hope this helps,