I don't think Lack is really fond of preferences, I wouldn't be either if I were him.
The problem with that idea is that you'll have to add something to everyone's profile, you'll have to maintain/check all versions of it when implementing new features and you'll have to produce the documentation to explain the differences (FAQ's, some questions in the forum will have to be answered, maybe something in the instructions). If you'd make everything an option, you'd very soon have something that's not really maintainable. And that would impact the speed of new improvements or fixes considerably.
All in all, I think this is too small to deserve to be an option.
Only if it really becomes slow in variant C (which I don't believe actually, since jQuery is currently used in
30%+ of the top 10.000 sites.. If it didn't perform for 0.5% of the people, it would never get near there), it would be an argument to make it an option.