The difference between having and not having the capstone while having all the +alacrity items is about 8 arrows/minute.
8 arrows of regular damage no crit/slay living/lightning strike using a lightning strike bow is about 8*(1d10 +20 +1d6 +1d6) or 8*{5.75+20+3.58+3.58} 263.28 extra damage from capstone. The numbers in the {} were gotten from rolling 100d6 in game and dividing by 100 to get an average number for the shock and goodburst damage.

So going pure ranger with the capstone adds 263 damage. This is assuming the mob has 100% fortification. The actual damage with crits/slay living and lightning strike would be much higher if only just one arrow was a crit.

The capstone + haste compared to capstone + haste + items(ToD deepwood sniper set) difference is only 2 arrows or 65 damage. So skipping items and just getting the capstone isn't that big a dps loss, though every bit helps with range damage being what it is.

For most end game raid content you will always have haste. If there is no caster they do make haste pots, but ud have to pause every 30 seconds to drink another.

I think that answers your questions