However, I will stand behind my comment. With every Android device I've worked with and reflashed (the list is pretty long now: HTC Desire, Z, HD, One X, Moto Defy, Atrix, ZTE Blade and an Ainol Novo7 Aurora) I can quite conclusively say that there are some (I'm not saying all) community-developed ROMs that run faster, more stably and give better battery life than the manufacturer-supplied effort.
I don't think the OEMs are idiots. There are a lot of factors at play here: the OEMs have a billion other things to worry about, and limited amounts of developers given a limited amount of time to work on each device. Plus the cost of developing and testing new software for existing devices, and on top of that - the demands of the carriers. I'd say they also have some commercial incentive not to optimise builds too much when updating older hardware - if the new hardware seems much faster (even if that comes from software enhancements), you're more likely to buy it.
Most of the components in modern devices are chosen 'off the peg' and manufacturers development teams don't necessarily know evey possible optimisation for the platform - as an example: the deeper sleep state enabled in QSD8250 on the Desire by open community devs made a massive difference to battery life at the time with no detrimental effects.
Anyway, if manufacturer shipped ROMs are so perfectly stable and functional and 'the best thing out there', I assume you're running a factory ROM with no other mods - rooted at most??? :P