How much battery would I need without sacrificing performance?
You are asking the wrong question. That's the best way to spend a lot of money, get no extra performance or range, and at the same time "over load" the lithiums so that they have a short life. Whats the point in this? This is EXACTLY what the manufacturers will do for the new lithium marketing, once they figure it all out. The answer is that 60Ah 24v lithium will give the same performance as your 100Ah batteries roughly, with absolutely no benefits in range, service life, and greater cost. (other than weight.) So why would you want them?
The question you SHOULD be asking is this:
Whats the MAXIMUM Ah I can squeeze into the space two antique lead bricks came out?The answer is approx. 150Ah. But this will give you around 2.5 to 3x the current range.
And because you have the same motor "load" (amps) on a 3x BIGGER battery size it has an easier life and is not heavily loaded. C rate is naturally about 3x lower. So service life, and torque is drastically improved. By maybe 2x. High currents kill lithium's faster.
And because your AVERAGE depth of discharge is now about 1/3rd of what it would be with a 60Ah battery, your cycle life will not be 1000 to 1500 cycles, but 5 to 10 THOUSAND cycles. And remember that as the battery is big, it may need charging much less frequently on average, so instead of daily, you may do this weekly... So lets say 3000 charges, weekly. That's an everlasting battery. Although shelf and USAGE life is still approx. 15 to 20 years for lifepo4 TREATED WELL.