205 x 65% = 133.25mm
215 x 60% = 129.00mm

difference equals 4.25mm . . . in theory. In practice tyres vary significantly between makes (as much as 4.25mm), and of course there is also tyre wear (more than 4.25mm difference).

i don't want to give any advice on this, but with a stuffed lsd i wouldn't worry about the difference, and it sounds like a kind of short term arrangement anyway.