I had a pair of Sid World Cup (Blackbox tech with carbon steerer). They flexed a LOT!!! However they came with the bike I bought (Top Fuel) and I didn’t really have the funds to change the to Reba’s, but I wished I could have.
The suggested air pressures on the fork leg only go up to a rider weight of 180lbs and I’m 200lbs. As I said, they did flex a lot, but after 4 years of year round abuse, here and abroad, (including a service once a year), they are now stuffed and it wouldn’t be worth servicing them again for the cost.
I never noticed the flex issue while riding (I had no direct comparison), apart from when appling the front brake and looking down, when the front wheel would seemingly dissapear underneath the down-tube!! OK, so that’s an exaggeration, but it did move a LOT!
The main reasons for wanting REBA’s was the recommended rider weight issue and the longing for a bigger front rotor. The SIDs are limited to a 160mm rotor and in the Alps I cooked the brake fluid all the time.
If you’re fat and need a big rotor, go for the REBA’s. IF your ickle and a weight-weenie, go for the SID’s.