I have and use the Voigtländer Collapsible Heliar 50mm f3.5 and will say that I have never used or seen any other 35mm lens that can resolve detail like this lens. The only lens that come close is the Zeiss 45mm f2.0 that was made for the Contax G-series.
Using the Heliar with Efke 25 or Rollei Pan 25, takes you into a whole different realm of 35mm photography. I will not even attempt to post anything here as my scanner is simply not capable of capturing the detail in the negatives. Every tiny detail is rendered in perfect, simply astounding.
I have always equated a 50mm with speed, having used and owned most of the 50mm f1.4's and 1.2's in may different mounts and brands, so it took a real mental adjustment when I started to use the Heliar 50/3.5. So I know consider my Bessa T with the Heliar 50/3.5 as my highly portable large format camera. To put it simply, with Efke 25 film, the Bessa T and Heliar 50/3.5 will meet or exceed must negatives from medium format. It is that good.
Also, I also agree that by the time you add a finder and hood, you lose most of the compactness that you would think you would gain with a collapsible lens. My Bessa R2 with the Color-Skopar 50mm f2.5 is actually smaller than the Bessa T with a 50mm finder and the extended Heliar with a filter and hood. But the Bessa T will travel in a more compact kit, you just have to do a bit of assembly before deployment, sort of like a large format kit.
Here is the Bessa T with the Voigtländer Collapsible Heliar 50mm f3.5 and 50mm finder, in the transport mode...
And ready for action...
I have found using a slow, but very high quality 50mm as fun and fascinating as using a high-speed 50's. While the lenses are the same focal length, in practice they are very different lenses with their own unique photographic challenges and purposes.