I doubt it...
U could make the comparison as the iPod touch 5 on ios 8 will be like the iPhone 4 on ios 7 now because the iPhone 4 on ios 7 has a 4 year old A4 chip...and the iPod touch 5 on ios 8 will then have a 4 year old chip....
But again, the a4 is a lot less powerful than the a5
And ios 8 will likely be different in computer consumption than ios 7...,
I think it may be a bit rough but I think it won't be too bad...
It should run well but I doubt the Touch will support everything in IOS 8.