Switch out the jockey wheels to Shimano Ultegra upper and lower 11t wheels to properly clear the 11-34t cassette. That should get the cage to clear that setup with very minimal chain slack especially when using gear protection. You can also get the 18t OSPW with narrow wide teeth to clear that combo. That combo can even clear 11-36t.
Thanks jonathanf2, I'm already running the 18t OSPW that came with the new cage and trying to figure out if that's what is causing the excess chain slack at when using both the smallest cog and chainring. Would love to hear if you have any experience in running a similar set up to me and whether or not you had as severe chain slack as me or if I've stuffed up the installation (either not set the derailleur spring tension correctly or if my chain is still incorrectly sized).
Thanks.