Limit Spotlight to mail

If mds is continually using that much CPU then something else is wrong, that's not usual behaviour. Once the index is created, it's incrementally updated and shouldn't be using that much CPU at all.

However, you could try telling Spotlight only to display Mail Messages in searches. You'll find that option in System Preferences in the Spotlight section. (edit) This doesn't actually stop Spotlight indexing those items, but it wont display them in results.

I think you have a more fundamental problem here with Spotlight that should be resolved. Try deleting and recreating the index with the following command:

sudo mdutil -i on -Evsa