Whatever crazy grade braking algorithm GM uses will brake up to 3K+, but Ive only seen it once. Tthats part of the reason the LBZs tac is different then the rest. GM got tired of calls from freaked out owners that their trucks where turning that kind of rpms.
They will grade brake all the way to 4700rpms before a forced upshift to bring the rpms back down but won't accelerate past 3250ish stock. That's why the tachs are like that. Then they changed it with the LBZ's like you said.