I wouldn't mind earning SPs quickier in the end game. It's really hard to get rid of bonus XP.
If the aim is to get rid of bonus XP, you really need to reduce the % amount of bonus XP given for many choices, rather than speed up the acquisition - although doing both simultaneously would also work.
Look at this way, as it stands right now, if I spend a SP that provides 100% XP back every time I earn a SP (say I have a nice cushion of 10 in the bank, and choose not to go below that), I will never run out of bonus XP, and it will continue to grow, as I earn 50% real XP, I use up 50% bonus XP, and now I spend that newly earned SP on another 100% XP option. Now I've got 150% bonus XP in the bank (growing to 200% after the next point, 250%, etc), and am spending 1 SP every 500,000 real XP earned.
Now increase the rate at which bonus XP is used, say make it 10:1. So I earn 10% XP, and that burn 90% XP. I spend a 100% XP SP. Now I've got 110% bonus XP remaining, and spend 1 SP every 100,000 real XP earned. The bonus grows to 120%, then 130%, etc. Bonus XP is still growing, despite earning SP at 5 times the rate.
The only way to have bonus XP truly guaranteed to run out, is remove bonus XP options above the gain rate. I.e. if you're earning triple XP, bonus XP cannot be above 66% per point. If you're earning quadruple XP, bonus XP cannot be above 75% per point, if you're earning quintuple XP, bonus XP cannot be above 80% per point, and so on. The closer you are to those break points but below them, the slower any built up XP reserve will drain out. Going above it just means faster SP gain potentially without reducing XP backlog.
The other way to look at it, is if everyone spent SP only on less than 50% bonus XP choices, you wouldn't have a backlog eventually in the current implementation.