There are more than one reasons pointing at this case. First acceleration is setup more demanding to the gpu. Quartz exreme and Core image are 3D/2D engines there are new in 10.5. Maybe you could change the software controlled parameters with CHUD tools (http://www.apfelwiki.de/Main/CHUDTools) But check the temperature with temperature monitor! In the ROM you may trace the Softrstraps responsible for Fan/Temp and hex it with Hexeditor. Follow the preliminiaries of Rom editing on themacelite.wikdot. But i am afraid of the softstrapped fan speed settings will be overridden by hardware respectively the OS. In this case you may mask the readings off the sensor?!