hmm...well if its affecting other projects, then regardless of whether the BOINC manager's assessment of 0.6 CPU per GPU task on your host is accurate or not, it seems those GPU tasks are actually consuming more CPU resources than they used to. do you frequent the Milkyway@Home message boards? you'll probably get more responses if you start a thread over there, and the responses will probably come more frequently too. a quick perusal of the number crunching sub-forum brought my attention to
THIS thread in which someone tries to address an issue where his GPU tasks are consuming 50% (0.50) CPU. he tried installing different drivers for his ATI/AMD card and that solved the problem. while i don't know if it'll do the same for you, it might be worth a shot.
also, try this: set MW@H to "no new tasks" and finish all MW@H tasks that are in the work queue. once that's done, detach your host from the MW@H project and then reattach it (be sure to report the completed tasks before you detach though!). see if that doesn't get the CPU consumption of your MW@H GPU tasks back to normal...try this before you try different video card drivers.