I deleted my ABS long ago for the simple fact that when tracking a car you bleed the brakes quite often either from replacing / rebuilding stuff or just routine maintenance before an event. The combination of the two means lots of bleeding in most cases. Bleeding the ABS system is a giant PITA - I chose not to deal with it any longer and pat myself on the back every time I work on the brakes.
In the 4+ years I've been tracking this car with C6 front brakes / stock rears I've never had an issue locking up the tires. That's not to say I've never done it but it has been surprisingly rare, always due to driver error (jumping on the brakes rather than squeezing them), and quite easy to modulate without ruining a tire or going off track. In fact I've never once flat-spotted a tire in all the years I've been doing this. I've also never ran a tire that wasn't a Nitto 555R2, Nitto NT01, or Hoosier R6. With street tires it would be a different story.
In wheel-to-wheel racing, I don't feel disadvantaged against cars with stock ABS. Factory ABS systems are legal in my class - NASA American Iron. Quick caveat - the Ford Racing ABS system is a whole different animal with too much internet drama already - I'm excluding that from the conversation. However, for cars running "stock" ABS systems, I brake at about the same point they do and at about the same rate give or take different drivers, setups, etc. Personally, I think ABS is a nice insurance policy if you'd just as soon flat spot a tire as put air in it but it doesn't make you any faster. It just helps you spend less money on tires. On a dry track, it all comes down to driver talent and car setup - specifically camber, shock valving, and to a lesser degree, spring rates. In the rain...yeah....ABS would be very nice to have but still requires driver skill to make it work. It's not the difference between mid-pack and podium...even in the rain. If it was, believe me, it would magically reappear on my car.
On the street - ABS can be the difference between crashing and not crashing. As absurd as it sounds, things happen much faster and with less warning on the street than on a racetrack. All of my family's street vehicles have ABS. We've been fortunate enough not to need it so far but I'm glad it's there in case we do.
You seem to have a car that's primarily a track toy with some street duty. I remember that period of my addiction with very fond memories

Since the ABS is already gone, I personally wouldn't go to great lengths to restore it but do practice looking as far ahead as possible, every time you're in ANY car. It's a good daily brain exercise for both track and street and lessens the chance of "needing" ABS. If you decide to significantly up the percentage of street duty, that's when I might look into what it would take to put ABS back on. Your financial and labor tolerances surely differ from mine so it's really a "choose your own adventure" book at that point....