People are for the most part....clueless.
XR6 is indeed a fine motor, as bullet proof as they come, but you need to understand it's happy place, and you and your motor will love each other a long time.
XR6 cannot by modified to much of any degree to get more out of it, Fact. Why? it all has to do with the port design, it's got little finger ports that won't enable you to get any more compression, or flow to it. Bummer, but it is what it is. You can install composite reeds and pick up a little acceleration, but that's about all it's capable of doing.
Now, rpms...the ignition module has a passive rev limiter, after 5,800 rpms it'll start retarding timing, you might be spinning faster, but you're making less power. So, you want to stay at 5,800 WFO. Whatever prop takes you there will give you the best results.
And...I don't like your oil pump to spin any faster than 6,000 rpms, that plastic gear isn't happy at that speed, and unless you want to disable it and pre-mix, you won't want to either.
Marble Falls/Seadrift TX