Experimented with this the past few days, FTB Ultimate. Built an auto-spawner with a 9x9 area surrounded by 4 grinders, one on each side to cover the whole area. The hardest part on my server - capturing a slime in a safari net. Once I had one, I used the slime in the auto-spawner to build up 2800 buckets of monster essence in a railcraft iron tank (turned out to be massive overkill but it only took a few hours to fill it, slimes are ridic LOL). Then I ran the essence upstairs to an Etho XP Blaze Grinder setup (see his great video). While the blaze spawner is running I keep the slime spawner going downstairs.
The blaze spawner cranks em out to the point I can't keep up and turn the machine off for a while to catch up. I use a lever above the liquid tesseract I'm using to send monster essence to the blaze spawner as the on/off switch for the whole thing.
So here's what I found. The tank never gets below about 2750 buckets. So theoretically about 50-100 buckets is all you need in reserve for this setup it appears. I also have an auto-enchanter running which takes a good amount of energy (I use my bank of 16 combustion engines running on fuel for all of this). The auto-enchanter is absurdly slow compared to just killing blazes (30 levels in less than 5 mins, maybe even less than 3 mins) and enchanting books on a table. It uses too much power and takes far too long to enchant one single book to be worthwhile. It also hasn't given me any good books with it set to level 30 enchants. Just the same books I get by enchanting them manually. For the time it takes to make one book in the auto-enchanter, I can make 5 books at my blaze spawner. Literally.
So the auto-enchanter is probably gonna go into a chest and gather dust (why waste that sizeable power drain?). The Etho XP blaze spawner is (as always) one of the centerpieces of my home base, altho I'm realizing in Ultimate that enchanting isn't nearly as useful. My main goal was to get Efficiency V on my Rock Cutter (which already has Silk Touch III of course) so I can speed thru collecting ores like always-in-demand Certus Quartz from the bottom of my quarries even when they're filled with water. So now I don't even need to make or repair a regular vanilla silk touch pickaxe anymore. But I'm still gonna crank out the enchanted books because they are a seemingly rare and somewhat in-demand item (by new players who can't enchant stuff yet, silk touch book goes for a lot of server cash) that I can sell on the SMP server I play on in a spawn shop. And as pointed out by others, the amount of drops I get out of the auto-spawner is just mind-boggling (more potential items I can sell in a shop). Skeletons drop huge amounts of enchanted bows (and no normal bow drops, go figure - with a manual vanilla grinder setup, the skeles only dropped regular damaged bows and almost no enchanted ones). With the auto-spawner, in one day I have over 2000 blaze rods. I had to void-pipe my slimeballs after I hit 6400 of those (a ridic amount). Etc. Any mob-dropped item will quickly be in the thousands if you spawn them for an hour or two.
tl;dr I use a slime spawner to create oodles of monster essence... then I use the essence to run a blaze spawner/grinder to turn the essence into XP. And don't waste your time with the auto-enchanter if you can build an Etho-style blaze grinder it's way quicker and better IMO.