I built this from a junkyard 350w currie motor with two 12v 10Ah batteries. It goes about 15mph unassisted and lasts for 8 miles w/o pedaling. The bike is an old mountain bike and pretty heavy. Weight is about 30 for the bike and 20 for the kit plus 190 for me. It bogs a little on the hills, but not too bad.
Can someone give me an idea of the increase in mileage a new battery pack would bring?

I don't know how used up the old batteries are, as I found them as is.

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this site will help you calculate what a new battery will do for you in terms of millage.
http://www.ebike.ca/batteries.shtml

you dont need to read all of it, just scroll down a bit and you will find a formula you can use. other simple things you can do to make sure you get the most out of your millage is keep your tire PSI the the recommended pressure, make sure your chains, gears, and wheel is well lubricated, as well as making sure your brakes are not rubbing up agesnt the rim untill you actually want to slow down.

as for the batteries themselfve, from what i see (im not sure about it though) it appears a 36v 10Ah battery will give more millage than a 24v 10Ah battery. so i guess the higher volt the battery the less Ah you need to get to where you need to go? im not sure on this, but its just something that i been noticing.

as for the battery type,

*Lead acid batterys- these tipically weigh the most out of the battery types you would use on an ebike and tipically will get less distance per Ah than other batteryis and have about a 200 cycle life.

1)short life span 2)heavy weight
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*NiMH batteryies- these are lighter than the lead acid batteries and have a longer life cycle as well and will get a better distance per Ah than lead acid batteries. expect 300-500 cycles from the batteries i have seen from this type.

1)decent life span 2)medium weight
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*LiFePo4- these are the best batteries you can get. they are more than worth there high price. they can easily reach 3000 cycles and can work in extreme heat and cold without any preformance loss. This is also the most powerful battery type you will ever use on an e bike, and yet it is the lightest of them all. this is by far the best choice to get.

1)super long life span 2)lightest of all the batteries 3)will work in exreme conditions