The Z550 is the one you want on throttle. It is faster and stronger than the Z270 and faster than the Z590. So it would be a nice upgrade for your throttle/brake servo but I would be lying if I said it would be a better replacement than the Z590 steering servo. Also the Z590 is metal geared with a steer servo that's a must imo and the Z550 is nylon but has a ball bearing.
Specs
JR Z270
Torque: 49 ounce inch (@6.0V)
Speed: .19 sec/60deg (@6.0V)
Weight: 1.50 oz
Size: 0.73 x 1.51 x 1.37"
Motor: 3-pole ferrite
Gears: Nylon
JR Z550
Torque: 62 oz/in at 6V
Weight: 1.50 oz
Motor: 3-pole ferrite
Speed: .17 sec/60 degrees at 6V
Size: 0.73" x 1.52" x 1.32"
Ball Bearing: Single
JR Z590
Torque: 85 ounce inch (@6.0V)
Speed: .15 sec/60deg (@6.0V)
Weight: 1.6 oz
Size: 0.73 x 1.55 x 1.46"
Motor: 3-pole ferrite
Gears: metal
I have a digital Futaba steering servo that will blow your Z590 outta the water let me know if you are interested in that, it does carry a beefier price but I am more than fair. I also have a digital HiTec high speed servo that will blow a Z550 outta the water so if your interested let me know. Digital servos are the cream of the crop, I cannot tell you the performance diffrence it makes.