Poor WiFi Performance on iPhone
Nothing to do with the router in the end. The source of the fault was a Layer 1 issue with the iPhone itself (D'Oh!): the case was absolutely CRUSHING TX performance as the screenshot after case was removed from the iPhone shows:

Performance skyrocketed immediately after removing the phone's case.
I always figured there would some degree of a hit from using a phone case on signal performance, but I was absolutely stunned at actually how much there was.
Anyhoo, hope this helps others improve crap performance due to these snazzy phone cases we all love so much ;-)-
The offending case (wood with some sort of rubberized material lining it) is pictured below at Dan's request: