The bushings on the diff are mainly to avoid or remove the "clunk" during shifting that is common from a loose diff mount. The ES bushing has interlocking metal pieces inside so the bottom bump stop is not necessary. The original reason for RT to develop his modified mount was because he kept breaking the stock mount, behind a powerful small block chevy engine. It's actually "wrong" for an L6 application when used with the overhead GM mount because it lowers the diff nose compared to stock. It fixes the clunk but probably adds a little drive shaft vibration. When it's used with a bump stop on the top and the stock mount on the bottom it's essentially just a movement snubber, not really a "mount".
The urethane bushings in the mustache bar also help with the clunk removal but are known to transmit diff noise in to the cabin. Many people go back to rubber due to the extra diff whine they hear after replacing the old worn out rubber. Beware.
Many people go to polyurethane because it's cheaper than the stock rubber. There's no huge benefit, overall, to converting from rubber to polyurethane. You might consider evaluating each step individually before going crazy.
Thanks very much for the insight. I have been going through this in my head, this will be a summer daily driver and occasional track car. I was worried the stock rubber wouldn't keep up its form with double stock power.
I am not using an L6, I am using a 1JZ . I I would have to check my diff angles but I can adjust the transmission mount slightly up and down...
My ultimate goal is a vehicle that handles well more for street conditions.
Hmm I think I will have to disagree on a couple points.
I am not sure about the RT mount, but at least the Technoversions style of it doesn't seem to lower the nose of the diff more then stock, I didn't encounter any twisting forces putting the differential in. Drive shaft flange was close to level or within the operating angle range for a driveshaft when I measured mine to get made, although I had my diff installed wrong previously the mounting planes weren't changed with the correction. With the mustache poly bushings it is pretty sensitive to deflections, I had to line up everything and jack it up slowly or it would bind. I suppose if you mounted the mustache the bar itself would be capable of deflecting quite a bit due to its construction.
I would say there is a fairly noticeable difference with polyurethane bushings in the mustache bar, I'll look for it, but there is a video of an EVO out there with a camera under the car and you can see the diff move drastically on power and off power, especially on shifts. In the scheme of things probably not a thing most people will notice when just normally driving, but I find that pre-movement a bit nauseating as the force comes a bit after the input.
I did the GM mount up top as the nose wants to come up when you are on the throttle the GM mount is supposed to be mounted to the bottom of a cross member so it is working upside down, while the casting of it kind of supports the tabs being pushed down when it is mounted correctly, mounted upside down the tabs kind of do bend away polyurethane with just gravity. I added the snubbing piece (good name for it) to give support while it is not under load.
I will agree though it would be a waste to poly mount things if NVH is your concern. Newzed is right, the poly kits are popular as replacing with stock bushings can be a real chore requiring quite a bit of press work if you can even find all the rubber bits. I have a mostly stripped interior, 3 inch exhaust with a bullet muffler, and a whiny fuel pump, so I have no reference for hearing diff whine really. I've added some sound deadening since then and I find it acceptable, but I have a daily driver so I don't have to live with the car.
I am rolling with a 3" exhaust with a muffler and resonance chamber. I am not HUGELY worried about noise..I just don't want the thing to Shake like hell... I don't want to go through the effort of pressing in new bushings and having a sore back every time I drive...