Have you even taken a look at the stats of grens and other line infantry? Grens have excellent stats, and they scale really well with vet.
There is abosolutely NO reason to add a 5th man on grens or increase HP/model...
Excellent stats man for man, but they drop DPS faster than any other squad in the game (keep in mind that poor poor tommies just couldnt survive with a .9 target size despite their reduced reinforcement costs, but grens can make do with a slightly larger one....) They also are very expensive to reinforce (more so than any allied counterpart) and are automatically subject to focus fire (5/6 men shooting at 4 men means its guaranteed than 1 gren model is being targeted by 2 or more men at all times. Meaning more chances to score a hit and more chances to burst down a model quickly) but they DO do alright in the early mid game, its late game where they are susceptible to wipes (all that delicately cultivated vet can disappear in a single roll of rng.. Or just focus fire from eaqualy vetted but better armed enemies) grens are just too squishy come late game and need a small something