Absolutely! I ran my Pentax MX (posher k1000 ;-) ) for years with a Tamron Zoom because I found the Macro facility useful. But I was getting more and more disillusioned with photography because the results were actually rather poor. A year or 2 back I rediscovered the standard 50mm lens that it came with and the results were a revelation! -all those wasted films because of that crappy Tamron (actually I think the poor quality was compounded by consistent underexposure- that wretched adaptal mount!)Then with the lenses I find there is a huge variation. With the Pentacon (Prakticar) lenses the pictures are OK but using a Sigma or Tamron the results are far better. I have a couple of Zeiss lenses that I attach to it also, totally stunning.
I don't notice the difference nearly so much between the Cinegon prime and the Optivaron zoom on the Leicina though- that is a very fine zoom lens!