Fattest doesn't equal most grippy. The difference in contact patch size (the bit in contact with the road) will be minimal. The wrong size tyre can cock up your handling, making you slower than on a sweet handling bike.
I had a 350 YPVS many moons ago which had oversize tyres on, as the smallest Hi-sports (stickiest of stickies, at the time) were bigger than standard, and I had to back off on fast sweepers, as it weaved so much. I was quicker on standard "unsticky" tyres.
Yamaha worked with the original equipment tyre manufacturer to ensure the tyres suit the bike, and as the bike handles well on standard tyre sizes, I'd look at getting the stickiest tyre in the standard size.
You'll be limited by what you can get on the 21" front, as you want to having matching tyres front and back if you can. (They suit each other). The front tyre should be the stickiest of the two aswell, as that's the one you want to start sliding last.
Something like the Michelin Annakee should let you get the pegs down.
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