Lifespan of 2011 iMac

I have a 27" 2011 (i7 3.4ghz 24GB RAM) iMac that I just replaced the drives (1 SSD and 1HDD) in 2 years ago. It runs pretty well now, but is just starting to slow down (and pales in comparison to my 2017 MBP). I'm considering dumping about $400 into it to add an SSD (and run both in RAID0), max out the RAM an replace the thermal sensor. I'm wondering if anyone has done any similar upgrades on a machine with close specs (specifically the age)? If so, did you get the boost in performance you were hoping for or is it just asking too much from a motherboard that old?
**EDIT/UPDATE**
there are some awesome answers/comments/info here - thanks everyone... much appreciated. My main goal is to have 2 high-performing machines. Like i said, I have a 2017 MBP that is great and this iMac is still pretty good for my purposes (no gaming or 3d animation). But once this iMac starts having trouble opening big Photoshop files while I have a bunch of files open elsewhere or performing operations in Illustrator on complex drawings (which I'm just starting to see the beginnings of), it's no longer "high performing". I really like the idea of the Mac Mini, but I can't justify the $800-$1000 expense. I could sell the iMac for probably around $500-$600 (and would have no problem justifying the $200-$400 expense), but then I'd either have to buy another large monitor or live with just 2 24-inchers... which would make me sad, I think. I **am** convinced that putting the $400 into the iMac is not the best idea. So thanks for helping me get there. Decisions. Decisions. Gotta mull it over a bit more.