It's interestng you mention that. It raises a couple of points:
1. 29.97fps NTSC frame rate
Just yesterday I was reading again about the NTSC 29.97fps frame rate. It all seems very complicated, ultimately to do with the limitations of physical electronic components such as frequency dividers, with the frame rate of broadcasts changing from 30fps to approx. 29.97fps when color/colour TV was introduced.
However, with digital media such as DVD and Blu-ray, I don't think it works quite like that, although I admit I am not certain. My impression is that, whatever the actual frame rate of an NTSC playback system is in real time, the in-built counter on the player always counts 30 frames for each of its 'seconds'. So for example, when the counter reaches 1:00:00, it will have displayed exactly 3600 x 30 = 108000 frames, and NOT 3600 x 29.97 = 107892 frames.
If I am correct, then for example a PAL -> NTSC conversion would convert 25 frames to 30, and not to 29.97.
2. Shriek Show DVDs
I don't know much about Shriek Show, although I do own quite a few of their releases. I had assumed that they mostly used source material that was 24fps scanned from film, with occasional exceptions such as 'Jungle Holocaust' that is a PAL -> NTSC conversion that converts 25fps -> 30fps, hence the shorter runtime (as it is effectively running at 25fps instead of 24fps).
If a 25fps PAL source is slowed down to 24fps and then stored on the DVD like that as a progressive transfer, there should be no interlacing issues. If instead it is slowed down to 24fps, then converted from 24fps -> 30fps using 2:3 pulldown, I believe some players can still reverse engineer this to display it as 24fps progressive.
But if many or most of their releases are sourced from PAL, that would surely compromise the picture quality even if the transfer is progressive, because of the different pixel resolutions. If an HD master is converted to PAL with 576 lines, and then from that converted again to NTSC with 486 lines, that would produce a poorer result than converting directly from the HD master to NTSC.
Having said that, I will still praise Shriek Show for releasing many excellent obscure titles, almost always uncut and with the OAR, as well as including many nice bonus features