I would guess that the undertray is common with the elise, so the oval cut out is for the normals elise downpipe's flexi.
IIRC the Exige has a janspeed 4:1 with flexi as standard. Janspeed make the flexi section if you need a spare.
As for fuel consumption, get the whole thing set up with verniers (if it's the 190 kit it should have verniers already?) and an emerald or DTA with wideband lambda feedback. Sure it'll cost you ~1K to get this sorted, but that's only 1000L of fuel - the engine should be able to return 35mpg on steady throttle cruise, but yours is probably doing 25mpg, so you'll get your money back in fuel after about 4,000 miles.
There is no reason for it to be lumpy on idle either - with the throttle bodies it has, properly balanced for airflow on idle, it should idle pretty smoothly.
Don't forget to rebuild the engine in 25K miles
Cheers,
Robin