I have the same iMac running 3.12/3.13 betas for several months now and can't confirm any of these.
Well, to be honest not 100% sure bout the bootlogo (would only be displayed for a splitsecond with my SSD) and I haven't really pushed it in the GFX department.
Unit is pretty silent with reasonable temps even when pushing the CPU (and no, no secret-handshake fan control active).
Bootlogo is minor issue. During boot process on iMac(HDD) I see grey screen and after that ambient screen.
Booting process with Powermac G5 Quad is faster (SSD), and I see the logo.
The fans on my iMac runs silently with MacOS and Linux. On full speed with morphos. In means it is probably no mechanical problem.
shows CPU temp 48C and other temps are lower.
Maybe Alert limit ? it's set to 0C for CPU and 80C for others.
AmigaOS: Pegasos II, Sam440ep-flex, Micro A1-C, AmigaOneX1000
MorphOS: Pegasos II, Powerbook G4, iMac G5, Powermac G5 Quad