VVT is mostly cam phasing, and that's not what your Rhoads lifter is doing. Tell me how much VVT is your Rhoads lifter doing at 2800 rpm and what makes you think is still not changing valve events at 5500rpm?
That's how the Rhoads lifters are designed. The lift is restored by the 3500-4000 RPM mark.
That said, ALL hydraulic lifters vary valve timing, since they are just that. Hydraulic. They don't vary it as much as a Rhoads hydraulic lifter, but there is variation. There is no way to keep that from happening since they are designed with a moveable plunger. The only way to take that built in feature away is to run a solid lifter.
