An expensive upgrade, but my vote for the #1, best, most-noticeable change to your ride is radial tires. I've posted plenty of times about it, the 'search' is your friend (upper-right corner of the page). Going 18" in the rear is less of a choice of tires, but you will be much-closer to your 15" 90 series tire, in height, and it won't cost you top-end, though how many of us have hit redline in 5th gear?
The radial tire switch is expensive because of the need to make a modified shaft drive OEM hub to a larger diameter hoop, 17" or 18". My machinist has made both.
The 18":
The 17":
Next to a stock VMax rim.
Don't mix bias-ply and radial tires! Yes there are a few bikes where the engineers actually did this, but they are the professionals. Even mixing the types of construction or the rubber compounds on radial tires can cause
evil handling! Some radials have the layers of construction at perpendicular angles, called 'zero degrees' while other radials have their layers at acute angles, much-less than 90 degrees (think of a 'V'). Choose one or the other.
Another consideration is rubber compound. Having a 'hard,' durable, long-lasting rubber compound on the front causes longer tread life, but less grip. That's not what most VMax riders want, unless you are a very-conservative rider, and you never see the oil red light flickering as you run through VBoost operation.
Run a harder rubber compound in the rear, for longevity, and you will find that even turning a right-hand corner, especially from a stop, and especially if the road surface is degraded, dirty, broken pavement, or someone's 'clunker' car just dropped ATF, oil, or hot tar like a roofing truck's tar kettle, you can easily expect the rear end to break traction if you are not very-cautious with your right hand throttle. Even crossing the black stripe of vehicle deposits in the middle of each lane at intersections can cause your tires to pick-up enough crud to cause you to lose traction after you cross that traction-losing stripe, and try to accelerate. Be careful out there!
All these considerations are magnified by precipitation.