The Roadster is a great GT car no doubt. I am a bit shocked at your MPG to be honest...pure HWY I will touch 27ish at sub-80 cruise speeds but getting into the happy pedal typically puts me in the 25 range. My combined driving for life of car has been 22.5 or so (25K miles).
With enough rain most any car get get a bit loose but if you are still on OEM tires, let alone < 1/2 tread I am not that surprised. The OE Sport Bridgestones were meh in rain at 1/2 tread or less. Newer S03's have *much* better rain performance, though it still pays to slow down
- b