Being somewhat taller than average, I always pay the bit extra for a legroom seat, (which has the added advantage of being (very) near an emergency exit!). I know that I have room to adopt an effective brace position. The reason for this is that on one of my first flights (about 10 years ago), the pilot aborted the take-off at the last moment, and stomped on the brakes (which he later explained was due to the fact that his seat shot back on its runners). During the stop, I was lucky not to hit my face on the back of the seat in front. Others were not quite so lucky.
Personally, I'd be happier with harnesses - I'm involved in rallying as navigator and safety provision, and I know what a big difference they make to survivability. However, I know that it is not really practical, so make do with what we are given.
Regarding taking luggage out of a damaged plane - I would not do it. Everything I will need from the minute I enter the airport to check in to when I get to my destination is on my person the whole time: passport, tickets, directions to destination, wallet, phone, keys (except when passing through security, when I tend to but everything except the phone in my baggage). Clothes with sufficient pockets are not hard to get, and I do not understand people who willingly separate themselves from these things. I might tend to be somewhat impatient with anyone p***ing about with baggage whilst impeding my exit from a damaged aircraft ...