As long as there's enough threaded body for the adjustable collar to be would down/tighter there's no difference between putting on a spring spacer and simply turning the collar down further for greater preload. The spring doesn't know the difference. Spacers are a band-aid lift for the older style shock units that have the short range five level stepped collar, they're a bit silly on a threaded collar type shock.
Both tighter preload and bracket style lifts have the disadvantage of potentially giving a rougher ride due to the fact that when the neutral point of the ride height is raised there's less available travel for the wheel to extend into a dip. This is more pronounced on a machine with a shorter suspension arm but its present in the wider models too.
Imagine if you had the shock units adjusted right to the hardest setting, so that if you lifted the machine off the ground the suspension woudn't extend any more. You'd have only compression available and no extension. When you encountered a pothole, the suspension would have no capability of extending a wheel down into it so the entire machine would drop into the hole and hammer you when it hit the far side. That's extreme but that's the principle.
If there's enough travel available to both lift the machine to some degree without losing too much extension capability and compromising the ride, you are golden.
Apologies if I am just stating the obvious.
