I use engine focused on ships I don't put weapons on as i know I'm always going to run in that ship. Some of the ships I've done that way can boost almost continuously. I like that setup for trader or exploration builds.
Otherwise all my ships just get the recharge rate mod.
I am sure that you are correct that a boost charge enhanced will allow for constant boosting, but I thought I might add that I added the level 4 charge enhanced (system, engines and weapons) on all of my combat ships, and garnered good rolls, averaging mid 20s percent recharge with small drops to the capacity as the offset. Even with my Mod3 level charge enhanced PD that I still have to upgrade on my FDL, the boost is practically constant with 3 Pips to engine, and 100% constant with 4 Pips. I think that even before engineers, the medium and large ships with I believe a size 5 or larger PD module would constantly boost with 4 Pips (maybe it was a size 6, but everything from the FDL and larger). Of course all of these mods are best applied when tailored to your specific play style, and with my play style, after trying both charge and capacity enhanced, I find that charge enhanced is the way to go over capacity, hands down... especially for combat. With exception to combat with smaller ships as a larger ship, it is unlikely that you will destroy a medium or large ship on a single full discharge of weapons capacity even with capacity enhanced. Since there is less waiting with charge enhanced, it will at least grant you more time firing and less time holding off for recharge, particularly if you have a balance weapon system or modded weapon system that allows you to fire for longer periods of time with positive charge (efficient). Just my opinion. So if you are thinking whether you should go charge or capacity enhanced, in my experience after trying both, charge enhanced is the better choice.