Not sure if most of the switching is between email accounts, but if it is, I've been using the 30 day free trial for MailPlane (http://mailplaneapp.com) and have been pleased with it.
I'm going to pay for it when the trial is over ($24.95).
I do have a weird 'lag' when I get to my e-mail in the morning after being away all night. My system does not sleep, but Sparrow is unresponsive for about 60 seconds. But just then everyday. After that I am fine until the next morning. I am not sure if that relates at all.
I have a lot of e-mail and 'labels' and I seem to do just fine.
TRIM support is coming in Lion (OS X 10.7) but I am sure you know that since you know 10.6 doesn't have it. I still felt compelled to type it though...
Not sure if most of the switching is between email accounts, but if it is, I've been using the 30 day free trial for MailPlane (http://mailplaneapp.com) and have been pleased with it.
I'm going to pay for it when the trial is over ($24.95).