Mansell was good enough to win the CART and F1 championships in the same year. He's also the fastest driver around the TopGear track by a whole 2 seconds. He's a quality driver, always has been, sure Senna got the better of him, but he was imo better than Schuey. Liek I said though, you can't accurately judge, I don't think Schumacher is by any means a second rate driver, either way it's close imo.
Well, here's what I think.
First of all, it is widely considered that Mansell won the 1992 championship because of a greatly superior car which had active suspension, abs, traction control etc. I myself didn't watch F1 back in the early 90s so I wouldn't know how the season went, but from what I've heard, this is majorly the reason he won that year.
Second of all, being the fastest around the Top Gear track means nothing to me. Schumacher hasn't participated and hasn't recorded a lap around that track. Anyway what does it matter? He was faster than Damon Hill and Mark Webber around the track. That seems trivial and has nothing to do with comparing Mansell to other drivers IMO.
Thirdly, Schumacher has raced against strong competitors such as Mika Hakkinen, Damon Hill and Jacques Villeneuve. They may not be the best, though I consider Hakkinen to be one of the best drivers ever, but they were still very good.
And the fact that he won the CART and F1 the same year doesn't mean much. Jacques Villeneuve won the CART championship in '95 and the Indy 500 the same year. He also went on to win F1 in '97. By those stats alone, they're "comparable". Schumacher on the other hand has won 7 F1 championships, and in at least 4 of them ('94, '95, '00 and '03) he did have a strong competition and not just him having a "superior" car.