On a 5 minute S3 raid where you are amazing and do 40% damage, a 16% damage boost for just yourself would shave less than 20 seconds from the raid kill time if everyone else play the same as before. Grind 15 raids, and you saved 1 raid worth to time. It will be even less in practice due to time spent filling boat, server failing ect. But lets just assume, for simplicity's sake, that for every 15 raids you do, you get 1 "free".
Just how many extra raid's worth of time it would take to pay for those mysterious shards? Even if the shards are only 10m each, you'd need to raid thousands of times before the "bonus raids" actually pay for the shards. New gear that will obsolete your old ones will likely come around by that time.
Shaving 20 seconds per raid is not "immense". You will probably not even notice it if you don't look at dps numbers.
It gets even worse on grind maps. You are one shotting trash with or without the limit release. Most of your time will still be spent from running from point A to point B, and not actually fighting. shaving a few seconds off a <1min boss fight isn't going to pay for itself anytime soon.
PS: unless the damage formula has been changed, your effective attack when capped is about 10k, with +900 from a fixed offset, +1875 from 300 additional damage stat, for a total of 12775. +2000 increase from limit release would end up being a 15.65% increase. The real attack cap is a little lower than +10k of monster def, so the increase from maxed attack limit release would probably end up at 16% ish.
The speed is more notable if you make a lot of mistakes or is unfamiliar with the fight; or if the boss goes invuln for long periods of time (like lako). The more optimized your gameplay is, the more strained stamina gets.
80 speed lets you mess up multiple times a minute and still use up your stamina. 50 speed would leave you with stamina to spare if you mess up a ton. On a near 100% uptime boss like havan, 40 speed would already get stamina limited with perfect gameplay.