An incomplete and mildly facetious list of possible solutions in increasing order of cost/complexity:
- Turn down the boost until the tires can handle it. Numbers on paper are just bench racing when unusable.
- Buy the stickiest streetable tires available, then return to option one and adjust accordingly.
- Buy the stickiest tires available, follow step one and avoid rain.
- Crank up the coil-overs in tribute to classic Detroit Muscle. Buy taller tires in accordance with option two, then follow option one. Examples: A 275/55-17 will add ~3" diameter (1.5" sidewall) over your 275/45-17 and will result in an effective rear gear ratio adjustment to 3.31:1 without actually changing the rear gear. 275/60-17 would add ~4.3" (2.15" sidewall) and result in a 3.19:1 effective rear gear ratio. Use a 275/45-19 or 275/50-19 (respectively) to get similar results with thinner sidewalls. The larger, heavier wheels should eat up some energy as well and further help reduce wheel spin.
- Change out coil-overs and control arms. Work with a professional suspension tuner to choose/customize the optimal valve tuning, spring rates, and alignment required to modulate the forces being transmitted through your car by the ~384% increase in torque over stock. Return to options one and two to complete setup.
- Change out the differential for one with a plethora of gearsets available. Keep original tire size. Follow earlier options as necessary.
- Mini-tub. Retain a low vehicle ride height and stock external appearance while gaining clearance for taller and wider tires. Follow earlier steps as necessary.
- Modify rear suspension design. Change out arms, angles, mounting points, etc. Target increases in areas like anti-squat to help drive the tires into to ground under power. Return to earlier steps as necessary.
- Back-half the car. Install a solid axle with either ladder bar or 4-link suspension. Follow earlier steps as necessary. Enjoy all the straight line traction
. Avoid turns!
In all seriousness, it seems to me that although tires are "always the answer" there is reason to question your suspension. You say you have your coil-overs as low as possible. If I'm not mistaken, a 1" rear drop on our cars results in 2-2.5* negative camber and Flex Z specs say they drop as much as 3.4". I don't know what the resulting camber is when that low, but certainly more than -2*. I assume your drop isn't the full 3.4" because of your tire width and rubbing, but it seems quite probable that you have more camber than you're estimating unless you have adjustable arms and have had an alignment. Weight transfer and minimal dampening will still compress the rear and add additional negative camber without traction, just not as severely.
Your cheapest option is to turn down the boost and/or re-tune the boost curve and traction control to better manage the torque. This will be the safest option for any road condition regardless of which tires you choose.
A cursory web search indicates the lowest ratio gear the is300 differential housing will fit is 3.54:1 (possibly out of a Tacoma or gen3 Supra, not sure I didn't read that close). You could confirm this and possibly find a usable used gearset to swap in for a few hundred dollars. Reducing the gear ratio will help prevent over powering the tires, but this is not much of a change and probably not worth it if sticking with the is300 rear.
If your car was mine, I'd turn down the boost on the street and keep a high boost tune for use on drag radials (or wrinkle walls) at the strip or for use if I ever went to a drift track. Then again, if I wasn't intending to build a dedicated drag or drift car, I'd never put that kind of power in this platform. But that's just me and I no longer have the same immortality complex I had when I was younger.