Depending on the MY, you may have up to 38Hp available when your battery is essentially 'empty' (but BMW won't let that happen). The EV motor is 170Hp...you will not have full capabilities when running only on the REx. Will the vehicle go? Yes. On the level, as long as it can overcome drag, yes...try to climb a long grade or have good acceleration, not going to happen at that speed. The conversion of mechanical Hp to electrical Hp is still Hp, but there's some lost in the conversion, so you don't really have 38Hp to drive the wheels. A 38Hp engine is sort of a slug. Reminds me of a 2CV, except the i3 weighs more.
The i8's motor is significantly larger, and can maintain pretty much any legal speed, regardless of the battery SOC. Not enough is known how they are implementing the new Mazda.
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above