Issue is not side washing. Bay is 30’ long. We use a longer HP hose already however any longer and the hose drags on the floor hence we are considering a longer boom about 12’ so it reaches closer to the front and rear of the bay.
I feel like you'd be creating a different problem with such a long wall-mount boom. If your trainable employees are going to be using the boom, that shouldn't be an issue, but if it's in a SS bay it's going to be very difficult for your customers to understand and figure out that if the boom is pointing toward the front of the vehicle and it's mounted on the driver's side (for example), they have to go clockwise around it with the gun to get the boom around where they can reach the rear half of the driver's side.
I've seen plenty of 15 x 30' wash bays with a 7-1/2' boom in the middle and there's no difficulty getting around a dump truck without the hose dragging the ground, but they usually have a pretty high roof.