I have a 10-gallon Polarware that I've used for about 100 5-gallon batches. It's awesome.
As for boiloff, I've taken some data points and then graphed them. They seem to fit a pretty nice linear equation. I tend to boil off 0.0227 gallons per minute. I just graphed all of my data points using Excel and then added a line of best fit. Put time on the x-axis, and your measured boil-off on the y-axis. Then choose "display equation" and the slope of your line is the rate of boiloff. This seems to hold pretty true for me.
That works out to about 1.36 gallons for a 60 minute boil, and 2.04 gallons for a 90-minute boil.
I was shooting for 5.5 gal batches, which means that I was typically collecting around 7 gallons preboil. This was never a problem in my 10-gallon Polarware. It's a great kettle that I expect to last me a lifetime!