What about the Z-Box option if you have an older boat? The guy I bought my 2003 LXi from was using it, and said it skied very similar to ZO.
The 2003 would either have the Monsoon 335 or the Hammerhead 375. The 375 obviously has more hp, but doesn't have any more torque. Malibu literature says the 335 has over 380 lb ft and the 375 has 387. I'm assuming the 2004 340 and 380 HH would be similar.
I would imagine the difference between the 2005 340 and 383 HH would be more noticeable though.