When I upgraded to 2.1 pentapods began expanding and upcycling themselves toward my spore cloud. Prior to 2.1 I had assumed they were unaffected by this mod, however 80+ legendary stompers/straffers a minute started plowing into my defenses about an hour into the patch and I discovered that strafer projectiles now do around 5k damage! It was glorious! My bots replaced hundreds of Legendary Tesla Turrets and thousands of cheap frontline turrets over the last few hours while I formulated a better solution.

For the moment I'm able to defeat their nests reliably without losing any of 20 legendary spidertrons, but if they have infinite damage scaling then I wonder how long until even my spidertrons start getting two-shot. I did have to control my spidertrons and do my best to avoid getting hit but it's hard to engage large bases without taking a couple hits. I'm around research productivity 50 which I suspect plenty of people would reach without being able to build and fully equip 20+ spidertrons. Further I don't see a way to defend against them without using large numbers of legendary Teslas and accepting attrition. Also, my agricultural science has been off this whole time and pentapods were still only able to expand into my cloud to the south of my base, so I have yet to see how many would be attacking when my research is active.

I think there probably needs to be mod settings for pentapods, though even just clearer documentation of what is being changed and how far it scales would help.

There were no changes in the mod itself with 2.1, outside of a recent bugfix of "pollution produced" factor not contributing to enemy research.

I assume pentapod attacks are due to 2.1 updating their expansion behaviour? From what I remember in 2.0, pentaponds could only expand to areas that had water tiles nearby, so landfilling marshes was an effective method of preventing their expansion.

You can go into mod settings -> map settings and tweak some settings to slow down enemy research rate, or set a cap on how much damage they are allowed to get - on default setting it is effectively uncapped (at max of 10k levels of +10% damage each). Everything has tooltips, so you can figure it out.

You've also mentioned "cheap frontline turrets" - are you playing with Modular Turrets by chance? Then maybe long-ranged stuff like Cruise Missile or Heavy Cannon turrets with explosive AoE ammo, quality and damage research investment would be a potential solution, to kill them before Strafers get into range? Cannon turrets specifically are a good candidate, since each Physical Projectile Research ups their damage by a whooping +100% per level.

If you can, a link to a picture(s) showing situation at gleba (or better yet, a savefile) would be helpful.

Oh sorry for the slow response, this wasn't intended as a complaint or bug report. I was unclear but yes 2.1 factorio changes caused my base to get attacked, and when I updated I already knew about the changes and thought I could get attacked.

I also missed the setting under map and only saw the startup settings. Does the research cost scale at all?

No, it doesn't. What scales is how fast enemies research, with most contributing factor being pollution. Default settings are meant for notable increase in enemy pressure for normal games (finish main objective, maybe play a bit more), with megabase-level of pollution output it gets quite insane after a time. You can turn on debug messages to get an idea how fast enemy research goes.