I know you said no multiple times on threads that are now closed, but we can't find no reason why this wouldn't be an improvement worth of including in the next version.
We did this in our install after seeing imports hang and never finish, and it doesn't seem to slow down subscriber inserts and updates. To the contrary, a simple index on the email field significantly speeds up CSV imports and subscriber additions (like 20-50 times the speed, for a subscribers table of 200k subscribers on an EC2 micro instance, from 3-5 subscribers every second to around 100 subscribers per second), and the list index significantly speeds up the Lists page (again, like 20-50 times or more).
It should also make subscriber imports and updates faster, since you locate the subscriber by e-mail, and without an index that will take much longer to locate.
(If I'm not mistaken, the only way indexes would slow down the app is if you indexed a field that needs to be editable. Subscribers' e-mails and list IDs never get updated, thus the index is not changed except for new records).