Hey Simaril,
First, you have to remember the vehicles like the Jagdpanzer, Marder, Hetzer, Jagdpanther, Jagdtiger, etc. used different tactics than tanks (when possible). Their tactics were "Hit and Run" or fireing from long ranges at unwary targets with their more powerful guns. That is when they did their best. Still dangerous at close range - the advantage started to go over to the tanks the closer the range became.
As you stated the guns on the tank destroyers were not fixed - the did have a limited traverse, elevation and depression. In fact if I am correct - the turrets on the U.S. M-10 JACKSON & M-18 HELLCAT TD's would traverse 360 degrees just like a tank & the Hellcat could do 55 MPH!.
On the German TD's - it varried due to the model but here are some of the major models:
T = Traverse, E = Elevation, D = Depression
MARDER III T: 21 deg L&R E: +13 deg D: - 8 deg
JgdPzr 38 HETZER T: 5 deg L, 11 deg R E: +12 deg D: - 6 deg
JGDPZR IV/70 T: 10 deg L&R E: + 15 deg D: - 5 deg
SdKfz 164 NASHORN/HORNISSE T: 30 deg L&R E: +20 deg D: -5 deg
JAGDPANTHER T: 13 deg L&R E: +14 deg D: - 8 deg
FERDINAND/ELEFANT T: 28 deg L&R E: +14 deg D: - 8 deg
JAGDTIGER T: 10 deg L&R E: +15 deg D: - 7.5 deg
Yea, it would be cool to have the Jagdpanther but I would much rather hunt the tundra for "Ruskies" in a G model Panther!
Even with the smaller gun. The 75mm KwK42 L/70 on the G was still capable of taking down the big Stalin tanks and litterly blew the turrets off the T-34's.