i have been educated as 1:4 (rule of thumb)...
the important thing is that it really should be rotational mass.
the more you shed towards the outer end will benefit more, that is, a wheel with 2lbs shaved from the hub will "feel" heavier than a wheel with 2lbs shaved on the spokes and barrels.
Again, these are all rule of thumbs. but i do agree that removing 2 lbs from wheels will have a bigger impact than removing 2lbs from an exhaust.
BBS FI FTW!