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Sliding out has nothing to do with how stiff the drivetrain mounts are. That's purely suspension. If you throw 12K springs and massive swaybars on there, then yes. The suspension can't flex and the tires will break loose. If you put solid drivetrain bushings everywhere and leave the suspension alone, all that'll happen is you'll take all of the compliance out of the drivetrain and greatly increase NVH. You may increase wear on actual drivetrain components because the housings can no longer move under load and all the stress goes on the components themselves. You might chirp the tires if you shift like a gorilla, because the drivetrain can't flex to absorb the shock of a rough shift.
Op, the second two letters of NVH are vibration and harshness. The engine moves around a lot. Putting it on sort of stiffer mounts such as Perrin or STI will minimize its movement at the cost of increased noise, vibration, and harshness being transmitted to the rest of the car. If you go with much stiffer mounts such as Cusco, it can quickly become intolerable. A lot of people who initially went with Cusco shortly downgraded to Perrin, STI, or OEM. Most people have only done engine mounts if they've installed an aftermarket header and overpipe and the pipes are hitting the subframe when the engine flexes.
The transmission is connected to the engine, yes. So I wouldn't do solid mounts on anything. You want a little bit of compliance in the system to account for shock to the drivetrain, occasional clumsy shifting, normal movement under load, etc... OEM drivetrain bushings are very soft because they're a tradeoff between effectively transmitting power to the wheels and making too much NVH for the occupants. It's up to you to decide if that line is drawn in the correct place for your own preferences. For me, it isn't. When the engine twists under load I can feel it in the shifter. When I shift I can feel the diff move around. I don't like that. The Whiteline transmission bushing insert (there are others, that's just the one I bought) fixed the first issue. The transmission can still move, it just takes more effort to do so. More power gets sent to the diff instead of being used to move the transmission around. I also threw in an Energy Suspension rear shifter bushing, because it's $20 and I was already under the car. I can't even say if it helped with shifter feel or not. It's stiffer than OEM, but how much did that part really move anyways?
For the diff, I went with the diff carrier inserts, but not the subframe inserts. Either one will reduce, but not eliminate, the OEM bushing compliance. Watch the below video; it's what convinced me to go with the diff carrier inserts. It's officially for Whiteline's subframe bushing inserts. Watch how the subframe mounting points move before and after bushing install. It didn't move a lot to begin with and now it moves less. Still moves, just not as much. Then look at the two big vertical bushings in the middle of the subframe behind the swaybar. Those things flop around like professional soccer players. Those two bushings and two smaller bushings at the front of the diff are the diff carrier bushings. If you shift heavily and you hear a clunk from the rear of your car, you're hearing the diff flop around on those bushings. Whiteline's inserts remove a lot of that compliance, but not all of it. Putting power down to the wheels has gotten much smoother since installing those bushings, at the cost of a little gear whine at low speeds. At high speeds road noise covers the whine. I didn't do both sets of bushings because I specifically wanted to leave a little compliance in the system and not significantly increase NVH.
[ame]https://m.youtube.com/watch?v=IDfJ_zYFPcs[/ame]
If you're after that super loud mechanical whine that you hear from actual race cars; what you're hearing are straight cut gears in the transmission meshing. Most consumer cars have helical cut gears. Straight cut gears are much stronger and can take a lot more abuse, which is why they're in race cars. They don't care if it's super loud and harsh, because performance comes before comfort. In consumer cars, it doesn't. Very few people would buy a street car that sounded like that from the factory.
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