No recals is a good sign.
Flat freeway driving will be almost entirely unaffected by losing the battery, at least in my experience. The places I've seen losses with mine removed are stop and go, driving in poor weather, and driving with lots of elevation change. Things that help are auto-stop, regenerative braking, and preventing the need of downshifting. Revving over ~3000 seems to tank economy and it's very easy to stay under 2000 with electric assist in just about any driving condition - low end torque is roughly doubled and it's virtually "free". Without it I often have to downshift sometimes 2 or even 3 gears when climbing any but shallow grades. But then, I live in a mountainous state.
In the flats you will have no trouble maitaining speed in 5th without assist. At 70mph you may see 60-70mpg with no mods or drafting, depending on the weather. 50mpg is more like 85-90mph economy.
No issues at all with normal operation without the IMA, other than that acceleration is much lower and you'll lose some economy in the above conditions. It's basically a gasoline car with an electrical assist added as an afterthought, the two systems are almost completely independent of each other.
You might be interested in some of the mods I've listed here since removing my battery:
https://www.insightcentral.net/forum...-k24-swap.html
Edit: Only major difference is the lack of an alternator - it uses a more efficient DC-DC conveter to step voltage down to 12v, generated by the IMA motor. When I took my battery out I replaced the 18 year old DC-DC with an even more efficient and much smaller unit.