My understanding of the FX8 is that it's primarily the preamp/special effects section if the Axe Fx. So, you would lose some of the power amp and speaker cabinet modeling. But, I'm sure somebody else who's studied it more can expand on that.
As far as the
cheapest way to get footswitching, the
Behringer FCB1010 is a surprisingly nice piece that you can get for less than $150 new. It's a
lotta gear for the money, and pretty robust/durable. But, I'll warn you ahead of time, unless you're
quite comfortable with MIDI and all that involves, getting the thing to behave well for you can be a real exercise, and it's still a tad limited. For all it brings you in capability, you lose in setup time. Although, to be fair, once it's set up you're good to go.
Another, and certainly better unit to consider would be
Voodoo Lab's Ground Control Pro. Here again, you'll have to learn a bit about MIDI and setup, but it does everything under the sun. Also very durable and robust, but they want $400 for that one. Fortunately, they're commonly on Ebay all the time for $300 or so new. In fact, I have a brand spankin' new one still in the box for $250 if you're interested.
Best choice, though, is Fractal's unit, the MFC101. I mean, it's built for the Axe Fx, so it's a plug'n'play unit. They're a little pricey, but they're also as sturdy as a truck and can work with a wide variety of other units as well if you wanted/needed to. Could be the last pedalboard MIDI controller you ever buy. Of course, you get what you pay for, so they don't give those little lovelies away.