you ve obviously not seen the size of my shoulders mate(im a bodybuilder too)! 
i doubt the slx poles will "kill me".your body naturally gets used to whatever poles your using.i will be using the extreme/ultimate brushes mainly and carbon goosenecks for some work.
Nothing to do with how much weight you can lift!
I'm at the gym aswell and that really helps, but it's rsi in your shoulders and rotator cuffs that is damage you never can really undo. The added flex and weight at 20feet high is what can do it over time.
I'm 30 this year and been wfp window cleaning my whole working life, and I plan to make my working day as easy as possible.
Extreme poles, electric reels etc.
About 4 years ago I was using slx poles and getting niggles in my shoulders, couldn't even bench press or do incline dumbells at the gym without a sharp pain. That was a wake up call, and since switching to extreme poles my shoulders have been fine since.
The amount of guys I see getting to there 50's and there shoulders fall apart and stop them from working is crazy!
If your in it for the long term then your shoulders will be the part of your body that lets us down.
It's worth the extra expense in my opinion.