I just tested pumping water from one tank to another and count the ticks it takes with combinators and stuff.
If I use one pump connecting both tanks directly, the combinator reads 181 ticks,
so I'd say the pump can theoretically transfer 25k of fluid in ~3 seconds which means 138.12/tick or 8287.29/s (The tooltip says 200 pumping speed, which apparently means per tick, but I guess the in-/outputs can't keep up)
Then I used one pump with a pipe at the input...
...and then 2 pumps:
I tried this with 0, 1, 2 and 4 pipes (pipe-to-ground counts still as 2 normal pipes[tested]) and later with more.
The results for one pump factor the tank in, which seems to count as something between 2-3 pipes.
When i used 15 pipes and one tank it took 1246 ticks (1203.85 fluid/s), so there it was ~3 pipes, but i haven't tested with different fill levels.
Since the pumps aren't the bottleneck anymore, it makes no sense to use pumps parallel now:
This setup takes 618 ticks to fill the tank, but if i remove the upper and lower pump, it takes 642 ticks, so it's really not worth it.
It looks like the sweet spot is 18 pipes between pumps for 1200 fluid/s (which the offshore pump can produce), so 18 pipes-to-ground would get you 99 tiles far.