I am told that a cart moving fast enough to overcome the viscosity of the magma and climb the exit ramp is also moving too fast to auto-load magma.
Actual testing is required.
I'm using an automated system based on impulse ramps to load and move magma around my current fort. No rollers, pumps or power required.
Empty minecarts enter from the left. They are accelerated with impulse ramps, climb up, fly over the empty space (covered by a hatch while the system is not in operation) and slam into the wall, losing all momentum in the process. Next, they drop into the magma reservoir, fill up and are slowly moved to the other end of the reservoir by the impulse ramps. This is where things get tricky.
Impulse ramps have just enough power to move the submerged cart vertically but they seem unable to accelerate the cart up the ramp at the end of the reservoir, no matter how many impulse ramps I lined up. The somewhat unintuitive but simple solution to this is to push it up with
another cart. So, to recap:
1. Cart(1) drops into the magma, fills up, moves up to the ramp and stops.
2. Cart(2) drops into the magma, fills up, slams into cart(1) and stops.
3. Cart (1) gets pushed up the ramp by the force of the impact.
4. Cart (2) moves to where cart(1) used to be and stops.
I've had this system running non-stop for about an in-game month at a time, with no problems. Haven't tried it yet but I'm sure you could incorporate the cart stacking trick for a rapid firing version.