I usualy use poly ur a thane ( 3m 5200) and fill the voids in the rubber mounts. works great. unlike the polyurathans hard shit sold by many that crumble when you relly dont want the to do so...( I had a red polly shifter joint crumble to dust as I was fixing to wax a stang with my bug....good thing traffic changed real fast as I only had 2 gears that worked.1&3...that would of made a mess on the road.......if we had gone for it. I was 2 miles from home witch was also good. ) I use the 5200 for engine mounts , trans mounts, suspension" compliance" bushings and what ever I need it for. it takes 24 hours to set up, so sometimes some way of holding it in place will be required,(tape,cardboard plastic etc) Ive even used it to make bump stops taller..permanent and dosent crack or degrade. small tube or caulking tube.they do make a fast set but I havent used it for this.( 3m 4200,both come in white and black)and some off brands., marine stores and some hardware stores.( my wife is a west marine manager so I get it there) keep up with the great work!!! do all the kappas have that type rear mount? looks just like my 2006 manual.
Kappa diff mounting styles:
2006-2007.5 all diffs mount to the body (different mount than what I show above)
2007.5-2009 all manual trans diffs mount to the torque arm (connected to transmission output)
2007.5-2009 all automatic trans diffs mount to the body (same mount I am showing above)
If you go back a few pages on this thread, I show a bunch of pictures of differentials and mounts
For the engine mounts etc, I have had good luck using a 2 part polyurethane with a duro of about 40, dye it black along with additives to make it last longer; and finally degas it before I pour it. You don't have to get fancy, filling the mounts with anything will help as you shared with us. On my Cruze when I discovered the insane wheel hop, I got aluminum scraps and stuffed them in the voids of the mount (temporary acid test), solved the wheel hop issue.
I have been blowing out kappa driveshafts (wearing out the ball bearings in the CV joint), the latest hypothesis suggests all of my stiff drivetrain mounts could be causing it (stiff engine mounts, stiff trans mount, stiff diff mount).