Hi again. I took a chance and upgraded to the U1n beta UEFI BIOS for my Z68 board. I'm happy to report that it resolves any problems with the 580 and BIOS splash screen. Have yet to test if it affects/resolves the issues I was seeing with the older bootloader. I now see the same splash screen as I do on my Z170 board! :-D Now to just get the system fully up and running and/or decide if I'll be sticking with UEFI or the formerly solid legacy f12 BIOS. I decided to give the beta UEFI a shot after researching how to upgrade from legacy to UEFI for my board. It turns out that there were prior beta iterations of the BIOS, like u1h and u1L, coming up in my searches. Basically, while labeled as a beta, the u1n iteration had matured a bit, something I hadn't been aware of prior to this. Here's to hoping it means a stable system experience.
Regarding getting everything working, I cloned my legacy drive and did a UEFI installation of Clover using MultiBeast. While I had selected the clone to boot from, Clover booted my legacy drive and it loaded up surprisingly without issue. Double checked my config.plist on the clone and noticed that it hadn't been updated as I thought it had been. Tried updating it and now the legacy and UEFI drives won't load past clover, just a black screen. Need to tinker with a default UEFI Clover config.plist to see if I can get my UUIDs and all that ported over. In any case, it's promising to see that my old legacy drive loaded up while using the clone with the UEFI clover install. Also, good to see that the upgrade for legacy BIOS to UEFI resolved the BIOS splash screen distortion with the RX 580!