I would guess that the issue with your Sonnensheins was, as you suggested, that they sat too long on the shelf without being charged. From what I've read, the design life for SLA batteries is much longer when used with a float charger.
Here's an example of a DIY LiPo conversion: http://andycaus.wordpress.com/2013/0...po-conversion/ With LiPo batteries, you can specify discharge rate. But it might be best (for the life of the flash) to keep the discharge rate within the SLA spec.
In the end, a lot depends on how you plan to use the flash. LiPo batteries might be best for infrequent use or if you walk around with the power pack. But if, like me, you're planning to use this as a portable monolight for amateur use, it's tough to beat the inexpensive SLA batteries.