The 6s ATX is fast. You'll have no problem dusting off the Yarises! At ZZL I was actually faster in the ATX V6 6s than in the MS6. (Very embarrasing, I know)
I'm not a fan of the manumatic because I feel there's too much of a delay between my command to shift and the trannys response, but then...
Helium is an inert gas and is NOT flammable.
Pure nitrogen is relatively inert too so it won't catch fire. Normal air is about 80% nitrogen anyway so your tires will not be significantly lighter.
Sweet ride
I love the 3 hatch. Love the color, your orange and 'winning blue'.
Does it have a trip computer? The kind that shows your distance to empty and mpg?