Members at 22rte-trucks.com or supratrucks.com, should be able to tell you more about the longevity of the r150 when running more hp than the stock 150/180 hp/torque that the 3.0 had.  While a few members her have done the 7mg and gte swaps, I don't know of any who have tuned it to the 300+hp mark, and I think most use the W series trans.  In the Supra community, the r154 is good for 3 times the original 230hp of the 7mgte, so you may be fine depending on the mileage of the trans, and where you start building boost.  Is the 300rwhp with just the basic power upgrades, or is that with running the Lexus v8 AFM, 550cc injectors, and 14-16psi boost as well?