I'm honestly just curious where on a public street you could ride a 250 hard enough to need to slide off the seat, knee out.
We play a lot of supermoto type riding on back roads with the dual sports and I do a fair amount of what would be high speed riding for a 250 when on my 550 and I've very seldom come across a situation where I need to hang off, maybe a knee out just for a minimal weight shift, but no sliding off the seat - just not needed, at speeds your 250 won't do in said corner.
I find it very hard to believe your technique on a 250 is remotely needed on a 250, unless you're on some go-kart track. The bike just doesn't dictate the need for it. So I have to question your skill at this point.
Go ride the thing without hanging off like you're at Laguna Seca. You may learn to have some faith in the tires.
I chased down some kid on a CBR 600 RR doing the same knee out half off the seat routine, riding my dinky 43 hp 550. I never moved off the saddle, nor did I even have to hang a knee out to keep up with this kid in the corners. He had form, but no idea when to use it. I saw the same thing where a woman was riding a BMW 800 on a winding road. I was stuck behind her and her husband/boyfriend. She was sliding off the seat to the inside of the turns at a speed my old mini van could do.
You've spent a thousand miles doing the wrong thing. Now go learn to STREET RIDE, not track ride. That bike will lean in so far it would make your head swim - without any hanging off. All that hanging off in your early learning is take attention away from actually cornering. Mike Hailwood did a lot of winning and riding faster than you or most of the rest of us without ever hanging off. You don't need to hang off until you go fast enough to need to gain the extra clearance or to avoid riding off the side of the tire. With a thousand miles on a 250 you are no where close.
Sorry to be brutally blunt, but unless you are the next coming of Valentino Rossi (and being fearful of leaning due to trust in the tires, you are not) you are on the wrong learning track. You are no where near ready for a 100+ hp bike. Cut the gymnastics on the seat and go with the flow of the bike in cornering, the rest will come later... much later.
As for trusting tires, the only way to do so is to simply lean in more when your head says not. Do it when you are play riding. On a clean known road, ride into a corner on the outside riding as if you were going to stay there, then intentionally move in tighter. I actually will goof around mid corner moving in deeper or out wider.
It enables me to have the confidence to do so. When I'm in a corner and I see something I need to avoid I can simply turn in deeper. And all this on dual sport 60/40 tires to boot. There are a few corners where I can drag a toe on a dual sport with 10" clearance. Practice and confidence.
Two friends who've done the Keith Code Superbike school said the biggest thing they learned was to be able to turn in deeper - the tires work better than the rider. Knowing riders, Code told them when going into a corner and thinking you may be in too fast, turn in deeper - lean it. The tires are better than the rider. I've heard of more riders running off the road than lowsiding due to the tire not sticking. Anyone who's lowsided did so due to road conditions, like sand or oil or pea gravel, not because the tire would't stick to good pavement.
When you learn and can do that, then go buy a GSXR.
so be safe and enjoy the new ride 
