If you're trying to replace a short riser with a little funny pipe (and we're talking inches and not feet), then pressure losses should be minimal through funny pipe.
As a guesstimate using friction loss charts for plastic pipe, it would seem that if you tried to force 20gpm through funny pipe (about equal to 1/2" pipe, perhaps a little smaller), your friction losses would be something on the order of 300 to 500 psi per 100'. So if your pipe is only 1 foot long, that's a 5psi pressure loss in the funny pipe IF your pushing 20gmp.
The bigger question would be do you want to try to force 20gpm through a 1/2" funny pipe. Because the speed of the water through the pipe would be HUGE, it would be better to use a larger pipe if you're trying to push that much water.
But unless this is a commercial install, I'm guessing your flow is going to be something closer to 10 gpm. At that rate, the pressure losses should be on the order of 80 to 200 psi per 100', so again, over a 1' distance, you'll only lose a few psi.
So the questions to then ask are #1, do you have a few psi to spare (or are you already pushing the design). #2, do you really want to push 10gpm through a 1/2" piece of funny pipe (because you'll be blowing the doors off that 5-7 ft/sec speed limit).
Can you find 3/4" funny pipe? If so and again you keep distances down to about 1', then pressure losses should be minimal and you might not be too bad on the speed limit issue (even at flows greater than 10gpm) just because the water is right at the sprinkler (i.e. the high flow at the tail of the pipe shouldn't cause water hammer).
Of course the other option would be swing arms, I know I've seen 3/4" material for that.