This is a very good comparison, if you want to check which Rim can fit what sizes of tyres: Rim to Tyre Application Chart
2.75*18 -- 1.40/1.60/1.85
3.00*18 -- 1.60/1.85/2.15
If the stock rear rim of GS150 is 1.60*18, then the maximum tyre you can put, is a 3.00*18, which isn't very good for a GS150, if you ask me.
I'll say it again. Go for a Disk Brake setup. You might have to spend nearly 20k on it, but i am sure you understand the difference it can make. Ofcourse you'll have to get bigger tyres. I'd say choose Tubless Radials.