I can tell you right now that's not the case at all. First of all, you are assuming an inserter bonus level of 7, hence not base speed.
Secondly, they are DEFINITELY faster than 2 stack inserters. Mine eats a 200 stack in about a second, my stack inserters take 6 items on this current game. That's far from 55 i/s to belts.
EDIT: If i were to make a guess, it would be whatever your stack inserter bonus is (my stack inserters take 6 items) every third tick.
Hence, at 20 cycles per second (assuming 60 FPS) you would see 6*20*2 = 240 items per second. The times two in there is because there's two inserters.
Alter the first value to whatever your inserters take and get a decent guess on how many belts you would need. Or drones.
EDIT #2: Though this is not in line with what Therax has stated. That a full BRL will transfer ALL it's items in about 5 seconds to a wagon at fill inserter upgrade.
So we can work backwards from that information.
A normal wagon takes 40 stacks, assuming 200 items per stack that's 8000 items. Divide that by 5 seconds and you have 1600 items/s. Each inserter therefore transfers 800 items/s at full upgrade. That's ~13 items per tick. Hence, the inserters transfer their items PER TICK going by what Therax stated.
Either way, you would need one 1600 item/s belt or two 800 item/s belts or four 400 item/s belts or eight 200 item/s belts... I don't know how many items/s bob's ULTIMATE belt carries but i fear it won't be enough.
Better use drones... How many depends on your upgrade status and distance between transfers as well as your ability to charge said drones when they run out of juice.
EDIT #3: Some further information... Due to game engine limitations, from what i've read, you can't exceed 240 items/second on any belt. ULTIMATE belts will let you do that so you need eight ultimate belts capable of 240 items/s to keep up with a fully upgraded BRL (again, assuming you don't upgrade beyond inserter bonus level 7) and that's also assuming you have a never ending supply of wagons to fill at that rate. Which you don't.
Why 240 items/s only on belts? Because the game places/draws items one per tick on belts. And miniloaders (which i assume you would be using) will only do 180 items/s.
So technically, the max miniloader feeding you can have on a BRL is 12*180 = 2160 items/s. Or if you get fancy and use the interface chests and ONLY have one BRL you can do 16*180 = 2880 items/s.
At some point, your inserter bonus (if you can research beyond level 7) will make any belt feeding setup insufficient.