2005 330i manual transmission performance package
Epoxying bottom plates as a preventive measure (no cracks present) isn't silly. If done right (using an appropriate industrial-grade epoxy), it's better than welding as it binds across the entire surface, not just around the periphery. There's a lot of room for arguing if this is enough or even necessary, not so much when it comes to comparing the overall strength of binding.I would avoid epoxy for fixing reinforcement plates. Silly half-arsed solution. If you're going to put in all the effort, might as well do it properly.
If you really want to make the car future-proof, you're best to also do topside reinforcement in addition to the bottom reinforcement plates. That would involve cutting up the boot floor and at the very least adding in weld plates that connect the RACP to the chassis rails. If you want to take it a step further, also add the weld in beam that connects the chassis rails for extra rigidity. Also, no plates are required there per se, but it's worth doing additional stitch welds around the trailing arm pockets.
No affiliation, but CMP Auto Engineering ones are what I'm going with for my Touring. Even if you're going with some other brand ones, their tech articles are a must-read to get a good idea of the nature of the issue. Most comprehensive write-ups on it that I've found anyway.