For starters, this post gives the recharge time difference between 0 and 100% of each statistic, which you can then extrapolate to see how much approximate difference you'll get, in terms of recharge time, by swapping certain gear.

In terms of "importance," I'd say it's a combination of personal preference (playstyle and neuroticism) and resources (time and upgrade materials).

To address your first question, it is generally advised to reach level 32 asap, since the increased light level (increased damage output and reduced damage taken) will benefit you far more than an increased int/dis rating (whichever you improve when you ditch the str gear).

As for the first example, again, that comes down to the factors above. If you're talking about raid armor, how many radiant shards do you have access to? How many do you still need to get to level 32? (I.e., if you find a better pair of raid boots, will you be upgrading it at the risk of not having radiant shards to upgrade the raid chest that you don't even have yet? If so, I'd strongly suggest you hold off on upgrading a replacement raid armor piece until you have a full raid armor set. (Because reaching level 32 is the priority.))

As for the warlock example, that 28 strength you'd gain on the upgrade works out to a few seconds of recharge time on your melee (based on the post linked above). Considering that reduction in recharge is quite small, and str isn't even useful, I'd skip that upgrade. (And hopefully you'll be finding better warlock exotics soon--that one is really a niche Crota HM item.)

Moreover, the cost to upgrade is 7 coins (for the exotic shard) and 7000+ glimmer. I'd much rather have 42 heavy ammo synthesis than 28 str, but that is based on my resources (I am starved for ammo synth) and my personal preference (I'm not that neurotic about min-maxing).

There are plenty of people out there who can't stand to not have the best item available (to them). Those folks would get the Light Beyond upgrade on principle.