Schiff did some tests a few years back, steeper is better. Problem becomes the stall speed goes up so you have to unload in the turn. Your stall speed may be above best glide in a 70 degree bank. They were only lossing 210 feet in a 70 degree banked turn. But he admits that's a bit too radical close to the ground and so came up with the 45 degree bank as a compromise.

Also you have less offset with a steeper bank so less altitude lost wiggling to to line up with the runway.

It's normal on a glider checkride to have a rope break at 200 feet and return to the runway.