Like you, I have a 2012 non R Spec 5.0, with the one exception being that mine is a very early 12' and accordingly, I do see a bit of oil consumption (aprox. 1 quart per 3k), which to be really honest doesn't seem so bad to me. Considering my 99 Miata ate about the same and my 400+hp DSM ate a quart per 1500 miles or so. I bought my Genesis with 42k 2 1/2 years ago and broke 108k this week.
Eco.... Still haven't noticed much difference with it. I do a lot of freeway driving and at 80+ for long durations, I still manage 22ish mpg. Around town, I see about 20ish, depending how much or how little I stay out of the fun pedal. The only big thing that I've noticed is that the light flickers on and off as I sit at around 80mph... (Eco turns off at 80mph.) so that being said, I've permanently turned it off.
In regards to intakes, the stock intake is surprisingly very good on the V-8. I made the mistake of buying a Weapon R "cold air intake" for my car and it made no extra power... In fact on the dyno, I actually lost power. I ended up reverting back to the stock setup and made a few
mods to it to include removing the air flapper at the bottom of the box, removing and blocking off the resonators and installing a K&N drop in filter. K&N doesn't specifically list a drop-in filter for the 5.0, but it's the same as the V-6. You want filter #
33-2426 .
As far as ditching the secondary cats, this is pretty easy to do and will free up a few hp, especially if you add an
aftermarket exhaust. The rear cats aren't monitored, meaning replacing them with test pipes won't give you any sort of check engine lights or anything. 3.8 liter test pipes from any of the coupe vendors will bolt-in, but I don't think that you will see any real benefit from them unless you back them up with an exhaust. I installed a Magnaflow 15059 system and have been really happy with the quality and sound. You're going to spend around $150 for the test pipes and between $1k~$1300 for the Magnaflow. Another option to save some cash is to take the car to a decent performance shop and have them weld you up a 2 1/2" mandrel catback system.
From the exhaust alone, I didn't see much of a gain on the top end, (maybe 10ish hp) but made a significant gain in mid-range torque and saw 10~15hp increase across the rpm band as well.
The sad part is that there is very little after market support for this car, to the point of being annoying. Have fun and hope that you've found some of the answers to your questions in this thread.