I have used Upstart for extra practice test. I felt the quant part of Upstart is representative of GMAT quant i.e. not easy quant as OG. However, verbal is totally offbase. In the two tests I took I got 650-690 and in my real GMAT I scored 760. So their simulator doesn't accurately predict your true score. In a nutshell, due to lack of data, noone knows how Upstart's scores shud translates into real score. We know that PP scores are fairly accurate and that one should add 50-100 points to Kaplan
scores. If you are getting high 600s in Kap then u are well on ur way to get a good score in GMAT. I wudn't worry about a 580 in Upstart. Take a PP test (before OG) and see where u stand.
Good luck !