You are missing out on quite a few things by not getting the premium package. I would need to go back and look at the pricing sheet again where it shows those dependencies. Have a look at one of the sticky topics for pricing and I believe there is still a PDF document that breaks it down. I used this a lot when I was configuring mine for ordering.
Regarding the instrument cluster changes with the NAV, you lose some enhanced display. Without the NAV you get 3-4 lines of information in the center between the large dials. It's all graphical regardless, so it provides a variety of information there.
If you have the NAV system, it extends this graphical display to right and overlaps with the bottom portion of the tachometer. In this extended display a graphical view of the fule efficiency and power regeneration is presented along with extended messages below the tachometer mostly for check messages. If you do not have HUD, I believe it displays some NAV information there, but not sure (I have HUD).
The owners manual may also give you a better picture of the differences as well with the display elements.
Are you trying to avoid leather and stick with leatherette? I did like the leatherette on my older 325 xi and it held up really well and great to clean. This is my first car with real leather and I like it too.
2013 X3 28i (prem, tech, cold1&2, conv, hud)