I agree with Ray about looking into shortening the tube is one better option rather than putting the mirror on 'stilts'. The longer bolts add their own flex into the collimation equation. This in turn compounds the tendency of coil springs to buckle, made worse by the longer span they need to cover. Coil springs are not unidirectional, and once they buckle (which is what you hear when you load them and the creek and groan), they effectively drop in the linear strain to where you want the load to go as they relieve the strain imposed on them out to the side. Heavier gauge coil helps, but the problem is still there and this is the main source of collimation movement during the night as a scope swings around and the springs are given a chance to release some of their strain, but just a heavier gauge lone is only half the story.
One way to control this buckling is the have the inside diameter of the compression spring fit snuggly around the bolt. This will reduce how much the spring can buckle out to the side. Trick is to find a suitable spring with a thick enough gauge to be stable and strong. Mass produced scopes have springs that are too large in their inside diameter, not even taking into account the wire gauge is too thin too. Remember, they are made to a price, not best for the job. This is why they are often replaced, but only replaced with 50% of their necessary properties (meaning the coils are thicker gauge, but the inside diameter is still too large so they can still buckle). In some smaller instruments, springs are done away with altogether and are replaced with a single O-ring!!! Then you cannot ever collimate the scope properly,
The only other way to improve collimation stability is rebuilding the mirror cell to beef up the entire unit and then be in total control of the collimation bolt diameter (work it out correctly and you also don't need a locking screw), and spring choice is more easily catered for. You then have the ability to use disc springs instead of coil springs as these are entirely unidirectional, so they never buckle and so always give 100% of their loaded strain in the direction you want, not prejudiced in any particular direction. Disc springs also help reduce the profile of the cell as they are very thin to start with.
Don't think of rebuilding the cell as overkill. Rebuilding it also allows you to set the primary mirror closer to the secondary as your original intention is! You don't need to cut the tube, and you can re-use the existing retension holes in the tube's base to hold the new cell in place.
This is what I did with my 8" f/4 Newt. I bought the OTA new, but the cell was damaged out of the box, so I got it for a bargain price as I was confident I could make a new (and better) cell. I used heavey gauge coil springs with it as I was not aware of disc springs then, but the coils fit nice and snug over the bolt - no locking screws here either. Massive bolt too, 1/2" - collimation doesn't move at all,
Raised the primary mirror by 2cm closer to the secondary.
Hopefully this gives you some food for thought.
Mental.