Bit-tech went and asked itself this very question, considering no amount of tarot card reading, crystal ball gazing or deciphering of entrails would provide a reliable answer.
To this end, the hardware review site not only reviewed Vista SP1, but vanilla Vista with hotfixes installed. This way, it could see whether SP1 was just a massive bunch of previously-released hotfixes, or if Microsoft had included some independent performance tweaks.To summarise its findings, there was a noticeable increase in minimum frame rates for Bioshock and Call of Duty 4, though only at extreme graphics settings for the latter. Crysis, World in Conflict, Supreme Commander and Oblivion failed to register increases worth mentioning. Essentially, people without the latest CPUs and GPUs will see the most benefit.
I'm not surprised really. Service packs are all about convenience (especially for IT managers) and security, with performance coming in third. If your PC has trouble running Vista now, SP1 isn't going to change that. Tech lovers should note that SP1 comes with DirectX 10.1, not that there's hardware to really support it yet. And even if there was, you'll need games to take advantage of it... if there's anything to take advantage of.
Windows Vista SP1 gaming performance [Bit-tech]