Well it turns out that the uphill is not so bad. If the grass is worn or not full, the robot gets limited traction, even with the spiked wheels. So sometimes a zig-zag pattern helps.
Where we really have a problem is when the robot follows the wire downhill. As we expand to multiple zones, we have problems with this. Our perimeter areas are hilly, and the Evolution can only follow the wire in a clockwise direction. Thus, following the wire downhill is unavoidable. This seems to be especially difficult on complex slopes - when the wire is not pointing straight down a hill. Luckily, the steepest of areas is in the last zone, so we can get the robot to come off the wire early... but there is always the possibility that, with a low battery condition, he will seek out the wire at the top of the hill and follow it downward.
With the downhill, I was hoping to use the zig-zag in a manner similar to traversing a mountain while skiing - softening the steep spots and limiting the speed by going across the slope. However, the zig-zag rarely works, and the robot spins out on the turns and goes out of bounds. Even a 20 degree slope is tough.
The only way I have found to deal with this is to keep relocating the wire until the robot seems under control on the downhill.
Overall, the robot does better with the spiked wheels then without them. I still want to try the wheelie bars - as there are some spots where he pops a wheelie - shuts off the blade, climbs a bit and then stops... then he turns on the blade and repeats the wheelie.