Oh boy, I want in on this! A lot of decisions surrounding upgrading to stiffer bushings relates to what you want to use the car for. If you don't race (or don't have the luxury of being able to drive very aggressively on winding roads) then there's really no point in stiffer bushings anywhere. In fact it's a negative for comfortable driving, just like some who modify their exhaust get very tired of the drone while on the highway!
You increase noise, vibration, and harshness (NVH) with stiffer bushings. The extreme is solid bushings where nothing shifts. It's not like soft bushings, which is like having a box in your trunk that slides forward and bangs into the back seat when you stop. Soft bushings slowly absorb the energy of each motion so you virtually don't realize the force that's being exerted. Whereas, with solid bushings, the mass is still there, wanting to move, but it is rigidly integrated into the whole chassis and never has an opportunity to move very far in any direction. The engine/transmission mass can (basically) only move the sheet metal attached to it. Of course they do it in pure race cars and often they have to beef up the mounting points in the chassis or subframes. As mentioned, once you start solidifying these isolators you start hearing and literally feeling everything that's happening inside the engine and transmission. Imagine having straight-cut gears! The torque and energy is still being transmitted in the form of vibrations which create a lot of noise.
BTW, In my pure track car I'm not out to win prizes/trophies. I have done some podium worthy laps, but I have left some things on the table for the sake of reliability. I use urethane everywhere; mostly SuperPro. Nothing is absolutely rigid except for my sway bar end links.
An interesting story: A guy tried to build a small sports racer powered by a snowmobile engine. It had no suspension! It was fast, but scary. It hopped around on the track at speed and vibrated so badly that his vision was blurred!
You don't need to understands physics much to realize that the energies created by moving mass have to go somewhere. That and the fact that a normal passenger car is greatly compromised for the sake of comfort. I chuckle when people make a car race-worthy and loud, but still spend huge sums of money on a high-end sound system.
Drummerboy, I love that double entendre, "don't look at them in isolation."