Hmph my bad. Yes its the Command Platoon OR the Leviathan.
Haven't looked at this list for ages
So my thoughts were that the Regimental HQ in the rulebook SteelLegion is +100pts in comparison to the Mechanised Company.
And 5 Fire Support units + the Supreme Commander generate 11 Autocannon shots which is quite some punch.
Regarding extra transports: This list is all about fluff. So one transport per Squad i would prefer to stand. I see the Leviathan as superior to 6 Chimeras.
I could go for this:
Company Command (400pts):
1 Supreme Commander unit
5 Fire Support
6 Chimeras
OR
1 Leviathan Command Centre (has Supreme Commander ability, see datasheet)
12 Infantry units
Note that this list was build with the last Codex IMperual Guard inmind where the Company Command Squad enabled to field 0-5 additional Squads (could be Fire SUpport, or Sentinels, etc for a total of 5 additional Squads/Squadrons).
@Leviathan transport capacity: I designed it to transport a full Infantry Platoon (13 units) with Ogryns and Sniper ugrades (each 2 units while Ogryns need double the transport space) if someone wanted to use it in another variant list.