no need to bore unless it is high mileage. bore only goes from 75mm to 75.5mm anyways, so that's no going to do anything for power.
You can't really look at it in terns of how many psi will I have to run with a t3/t4 turbo to hit 300 WHP.
It all depends on how efficient and built the rest of your set up is, and the biggest factor is tuning.
Things that will make a difference are engine compression, intake manifold (we still don't know what D15 it is- if it's not a JDM D15B then don't bother reading on... lol), Manifold design, trim of turbo, downpipe design & exhaust dia., muffler type, intercooler & piping dia. head work and cam, fuel & engine management and tuning. There's a lot of factors, but in a nut shell yes it is possible to hit 300 on a built sohc with stock sleeves if the bottom end is built for it (don't forget clutch too) and it is tuned well
