Let me rephrase. Grens biggest weakness is close range CQC combat. Right now Grens overall are bretty good and it's done well for making Ostheer early game less brutal. The issue with giving them potato mashers would be 2 fold;
1. Making them much much better in CQC, and making it harder to exploit their main weakness
2. It would give Ost to much building clearance on top of the already excellent Ost mortar making building maps hellish for allies.
Rifle nades just need a change to be not able to fire at full range while suppressed and not have crazy insane vet 2 accuracy. We don't really need anything drastic here.
gren's weakness isn't in them being poor at close combat though, it's that that's the best range for most of the infantry they'll face.
removing the RG and replacing it with a hand grenade would make grens stronger at close range but it would drastically weaken their long range and the increase in their close range would be negated by player skill and attention.
i don't think grens need to have the RG swapped with an HG (changes to the RG would be enough) but overall such a change would be a nerf to grens, not a buff.