Legend: yes, such an option would be a good idea. I have thought about that, I think I have not added it because command line options are undocumented. Maybe I should just document them (and also the option to change the geometry by pressing Space, 4, 6, or 8, and what's the geometry about in general).
Bear, I probably should test it a bit more to be sure, but I think it's the same things that happen when adding new races or classes: they give advantages and disadvantages (both in terms of challenge and being interesting in general), and it is usually not obvious whether the net effect is positive or negative. If it turns out to be seriously negative, then we can always tweak the game in order to remove the effect. Whether it's race, class, or geometry, it's the player's preference of what style they choose. Options do not have to be equally challenging, they just should all be interesting.
While consequences of getting caught in the open do matter in Hydra Slayer (I remember situations where I have been attacked by several hydras at once and had to use interesting strategies to deal with that, like stunning some of them with powerful weapons so that they act like roadblocks), I think the challenge in Hydra Slayer comes mostly from (on low level) using the equipment you have to slay hydras effectively, and (on high level) creating equipment which allows that. Both of these are not affected by geometry (except if you are a heavy user of missiles), and thus I think that game remains good after changing the geometry, you just lose some things and gain some other things.