In essence, yes, as dependant od bed volume vs flowrate vs total system volume and of course po4 concentration, po4 limitation is theoretically possible.
I doubt wheter this will be the case, as even though Orca phosphaguard is very powerful, i doubt whether all po4 will be removed at the flowrate over such a small bed volume, leaving some to provide bac with the required quantity for assimilation into biomass.
Phosphaguard will however need to be removed for rechargeing/replacement prior to the cubes, which in this desingn will be a bit messy.
Or should i rather say less messy if po4 media was last in line. Regular "massaging" of the gfh will also increase effectiveness, which seems a bit of a schlep in this configuration.
Also, fines coming off the gfh could clog pore structure of cubes. What effect this will have on the growth rate and consequent removal of nutrients is a bit above my level of overstanding...
There is however another reason.... The order of filteation, i.e mechanical, biological and lastly chemical, simply to reduce pressure / increase lifespan of chemical.