I agree that subtracting the TW, while legal is not the way it needs to be, and is the cause of many axle and tire problems we have.
If you are looking to upgrade, then an axle swap done in your driveway is not all that hard or expensive. I would suggest skipping the 15" tires and going straight to 16s. Most of the best tires made today are only available in that size.
Get 6 lug axles when you swap and rims to match. Some LT tires and you will have a signifignantly better setup than you have now.