I put in 36 of
these in my 24 x 36, on a 24 inch spacing (conveniently each rafter.)
Claimed lumens for each fixture are 8640 which may be optimistic but I've certainly meet my target of a minimum of 100 foot candles.
Attached FileDon't bother with the old fixtures.
The 8 foot units are really light, mount with three screw in clips and can be chained together if you like (I didn't). I ran conduit and four circuits in a staggered fashion so I can run 9, 18, 27 or 36 lights.
The lights come with a pigtail, a short and long daisy chain cable, so I got 3 prong plugs for them off of eBay. I figure it was worth the time to run outlets so that in five years when LED output doubles I can pull stuff down and replace it.
ETA: If you can rent a lift, do it, otherwise a scaffold with wheels on it makes things easy. I used two ladders and it was only a modest pain in the ass with 10 foot ceilings.