We used to achieve 10e8-10e9 particles/mL and the protocol that we had is identical to the one that Konstantin details in the thread linked below.There is no MW cutoff, maybe you are confusing ultracentrifugation with untrafiltration (Check Alexandra's reply)?

https://www.researchgate.net/post/What_is_easy_way_of_concentrating_lentivirus_particles_and_is_there_any_way_to_titrate_lentiviral_particles_without_kit

Whether or not you can achieve high titers depends on multiple factors, the main one being your transfection efficiency.

Thanks! I was confusing the two methods. Thanks for clearing up the confusion.

Is it common to check the transfection efficiency in the packaging cell line before producing virus?

We used to have GFP in our lentiviral constructs and never tried concentrating virus unless >80% of the HEK-293 cells were green after transfection. This is critical when you are trying to achieve high viral titers. If you have a low transfection efficiency, chances are the viral titers in your harvested supernatant would be low.