I think anyone who wants more upper-mids should start with the bass and treble at 9 o'clock, then adjust mids at 2.5khz until they get the presence they want. After that, you can season the bass and treble to taste, keeping in mind that too much bass will smother the mids, and too much treble might sound brittle for some.
Also, since the treble control isn't centered at 4k when boosting like on the previous heads, which happens to coincide with tweeter crossovers in many cabs, then you can get very different results by varying the tweeter level on your cab in conjunction with the treble control. For example, with my Shuttle 9.0, I'd usually run the treble flat and have the tweeter at 9 o'clock. With the Streamliner, I find that I like the treble at 10 o'clock, but the tweeter at noon. If I run the treble flat, then I pull the tweeter back down to 9 o'clock, but that sounds completely different and is more glassy.
Since they Streamliner's treble starts to cut like a passive tone control, you can really pull off that top glass, then crank your tweeter a bit to compensate, which allows the very top of the those upper mids to come through (if you consider 4khz upper mids...not really, but it's in the "presence" range).
Also, I do not think upper mids are missing in the Streamliner. Again, if you've seen a scope of the Carbine or Shuttle heads set flat, then you know right off the bat that upper mids are boosted in relation to everything else on those heads when set flat. In other words, the default voicing of those heads is upper-mid dominant. I still haven't scoped the Streamliner yet, but my guess is that the default voicing of this head is low to low-mid dominant, but that doesn't mean the mids are "scooped" or missing. Compared to a flat signal, I don't think either the Streamliner or the Carbine/Shuttle heads are far from that, but compared to each other, there fairly different.