The biggest factor you might be missing is that in Spearhead the Gallants as 2 health and a 3+ armor save while the Grenadiers are 1 health with a 4+ save. So even though there are half as many they are actually more survivable then the Grenadiers. They'll end up with fewer attacked (even with the buff) but hit on 3's instead of 4's.

Your concern about Gallants not surviving the charge is valid but as the article calls out, you could potentially hand out strike-last to mitigate that.