Chester (and more often than not, the band in general) usually sounds like shit on webcasts. I don't think there's ever been a KROQ performance where LP truly sounded "good" on the webcast recordings. Compare the audience recording of KROQ 2003 to the webcast, for instance. Night and day difference in terms of how they sound, despite being the same show.
Compared to a lot of his peers (and by "peers" I mean singers who have been performing for 10+ years and do a healthy amount of screaming), he's probably one of the better live rock vocalists. He's far from perfect, but that's to be expected.
As far as modern rock goes you have to give him a lot of credit because of his range. He really does push his voice more than anyone else in the genre. You could argue for Corey Taylor or Dave Grohl but other than that he pretty much stands alone.
Regardless if the webcasts make them sound crappy or not, its quite evident in tracks like CASTLE OF GLASS and Given Up that Chester was doing pretty bad.
I've always thought Dave Grohl was terrible live, actually. He over-sings/screams a TON (there's a difference between singing aggressively and incoherently yelling without any regard for pitch - he does the former on albums and the latter live) and usually completely loses his voice by the end of a set.
Having gone to soundwave, I can compare a few vocalists now. Chester was as good as any that I saw. My girlfriend even commented on how good LP sounded and she is by no means a fan. She said "they actually sounded a lot like on the cd". Danko Jones of, well, Danko Jones, was awesome. Very strong vocals, and that man knows how to get the crowd on his side. Both Mark and Tom of Blink sucked. I went in thinking that Tom's voice was weak and he'd sound crap but I never expected Mark to as well. Very disappointed with these guys. Biggest disappointment of the day. Deryk of Sum 41 was pretty good. By no means a weak performance. Loved their set. James of Metallica was good. He delivered a rock solid vocal performance. You expect no less from him though. Corey from Stone Sour was great too. Not perfect, but great listening. Really gave it his all too. That's what you want to see. There were a range of other bands I sort of heard in passing, but none that were bad or great. Although, Billy Talent seemed to have the crowd going off.