I saw your tooth on your website. Nice idea. With the quick change it would be useful to have especially for ripping out smaller stumps. I don't think it replaces the heavier duty ripper on a backhoe however. While there are advantages to a ripper on the FEL there are disadvantages too. Backhoes generally have more power than matched tractor FEL breakout for starters. I know my Woods BH90x with >5000lb digging force is more powerful than my 3700lb breakout force with my loader for example. YMMV.
It would also be harder I'd think to maneuver the whole tractor around a tree in densely forested areas as you need to get the whole tractor around the tree pretty much. With a BH you can just reach the arm behind the tree while the tractor is seven or even eight or nine feet away. I move the BH 90 degrees once after initial positioning to get all four sides of a tree. Using the FEL mounted ripper would be more like using a grapple where you are constantly moving the tractor and taking bites as you go. Surely for small stumps the FEL would be faster but for taking down whole trees, especially in a dense stand, I'd put my money on the BH mounted ripper. You might have trouble getting very close to a standing tree as the width of the FEL arms interferes but then again if there is space to maneuver around an isolated tree then you can just drive around the tree and get it from different angles.
Your FEL mounted ripper is limited to digging the length of the ripper itself as the FEL mount will hit the ground if you try to go deeper. With the MIE ripper the ripper clears it's own trench as you rip so the next pass down will be eight or ten inches deeper and the depth limit is the same as for a backhoe bucket or about six to eight feet, certainly enough for any stump.
Having the ripper on the BH leaves the FEL free to carry trees and stumps with bucket or grapple. With the FEL mount tied up with the ripper tooth on your device you either need to switch it out every time you want to move a stump or you need a BH to awkwardly carry the stump around. Maybe not a problem if you just pile up small stumps where they were removed and come back to get them later with a grapple.
I'd love to see the FEL mounted ripper in action though and imagine it would be ideal for cleaning up stumps after chainsawing stands of smaller trees.
And, yes, Spruce stumps are not a big challenge. Soft wood and relatively light. I'd prefer to use the ripper but twenty minutes with a grapple would do it too.