They are using a 4.2V. I will test mine with alkaline, a LifePO4 and then a 10440 and see if there's any change in output and post it. The LiFePO4 is basically just like using a couple of lithium batteries (standard non-rechargeables) but their capacity is pretty poor for the AAA. The ones I have are only 260mAH. The AA ones are 700mAH. 10440will generally be around 600mAH. It's what bothers me the most about using LiFePO4. Capacity stinks for the AAA.
Standard AAA alkaline 82mw
LiFEePO4 91mw
10440 101mw
So yes, as others have seen, voltage matters. Most pens like this are VERY stable and seem to have no ill affects (at least from my limited tests) from the extra voltage. Is it worth it? Hard to answer that. Every pen will react differently. Some may have very little increase or may actually drop in output while others may give up to a 50% or more improvement. All the really nice expensive pens I have from places like laserbtb (SKY) have a HUGE increase in output going from (2) to a single 10440. You should assume that the lifespan of your unit will go down. How much? Not a clue. It's a risk assessment you personally have to make.