I filled my CRX mounts with silicon...worked good, nice and solid.....I'd do it again if I had another honda.
As for durometer....to hard should not be a problem. only thing I can see is the the chain reaction caused by tightening something up.....if you tighten the top up...don't forget the bottom. if one side is tighter than the other...then equal them out. one of the biggest mistakes done to cars is slapping a new head on an old block...the rings are not used to the new pressures..and they blow, put a strut tower bar on the front, and keep the same old suspention on the bottom...and something is going to give.
I've played around with variouse compounds for durometer wise...normal silicone is soft, flexible, and stable...even for torn worn out bushings...I'll just fill around it, and beef it up..it's lasted 20K miles..no problem.
I've also experimented with aircraft fuel cell sealants...and so far I've had nothing but good results. It's a little expensive...but way worth it when it cures...the stuff I use is called pro-seal b-2...and it rocks...it's harder than poly, and it's resistant to almost everything...fuels, detergents, solvents.....but I haven't had a good test on heat...but...most motor mounts are away from the real high temps anyways...
it can be found anywhere a plane is kept....it comes frozen as a premixed form, but also in a two part mixable epoxy.......and we use it to seal everything....not only for fuel tanks...but everything. IF there is an exposed bolt or nut to the weather elements...we'll slap a glob on it...and it keeps all the harmfull corrosion at bay, plus we seal up the helicopters with it to make them watertight....It's awesome stuff, but be carefull with it...once it gets on you...its one you a while, so wear gloves when working with it, or keep your hands dipped in a soap film...then you can spead it around and get it into places you thought was imposible.
there are two type of proseal...the B 1/2 and the B 2....the B 2 is the 2 hour workable type, and the B 1/2 is the 30 minutes workable type. If you have no time to wait for it to cure, then get the b 1/2....and I also found that setiong the sealant in hot water before use makes it a little easier to spread and it cures faster.
as for the longivity of this stuff...since I've been around it, I haven't seen it break down, but I have seen it crack a little in high stress and vibrational areas...but for a motor mount...if you fill it in properly, and fully...then it wont see the real forces the engine gives it. and if it breaks down in 5 years...then just hollow out the stuff with a drill, and re-fill it. if the car lasts that long.