The new projects with their higher memory demands are more likely to affect the average computer.

WCG won't steal a single clock cycle that is needed by another program. However, there are other ways that grid computing can affect a computer. Mostly this has to do with IO, and under some circumstances a slowdown may be observed.

As far as overheating is concerned, the 60% is a safety margin. This margin is helpful for attracting large corporate networks to the grid.

Some of the technologies that the throttle was designed to cope with didn't exist when the grid was launched. WCG are merely responding to new information and new technology.