Hello chipsneeze, welcome to the forum.
1) Midi to computer with some high end VST samples and some new external speakers. I have a spare laptop that could always sit on the YDP-161 so it could work nicely. What do you think? Is the sensitivity of the controller good enough to drive the higher end VST samples?
Yes, the sound produced by a VI will likely be far superior to the built-in sounds on your YDP-161. Some developers offer trial versions for testing. If you have haven't done so already, I recommend giving Pianoteq a try. The trial version has some notes disabled, but other than this, the software is fully function and therefore perfectly good for testing. Some players really enjoy playing Pianoteq, others do not appreciate the tonal quality. However, there is no doubting that it is a very well developed piece of software that improves with each version.
2) Firmware updates or inside guts replacement -- I'm guessing after a few google searches that this isn't possible. But I'm a bit surprised.
Most digital piano manufacturers offer firmware updates for their products that fix bugs and occasionally add new features. However, changing the built-in sounds is rare, while no manufacturers (to my knowledge) offer customers the ability to upgrade pieces of technology with new/updated hardware.
I hope this helps.