The grid screen is to keep chunks from damaging the MAF and a mechanically failed MAF from entering the engine. I doubt that much air flow is lost and you probably have other restrictions in the system to get rid of. Dig around on the Bosch web site, I've seen flow charts for an assortment of MAFs.

Also, the inlet side grid would tend to straighten out air flow so the MAF sensor element gets smooth air. Remember, most MAF units sample just a portion of total air flow and assume the rest based on MAF housing diameter.