I've recently been reading the new Wrath and Glory RPG and in my own efforts to convert Rogue Trader, I've become more and more convinced that large piles of XP aren't really the answer if you want to play a higher power level in the WH40k universe, it is more a matter of narrative and scaling up/down enemies to suit the challenge. But it is important to have more 'toys' for your character to play with to make them feel like they have accomplished a fair amount.

Here's how I've done the Tier separation, but bear in mind that it is very rough and may require tweaking:

Tier 1 ( Your Acolytes, Imperial Guard, Gangers): +0XP to the starting from their Home World archetype; max skill rank: 2 (see @Tom Cruise 's Dark Heresy conversion)

Tier 2 ( Veterans of the above, crew of a Rogue Trader ship): +100XP for career skills; +50XP for non-career skills; +30XP for talents; max skill rank: 3

Tier 3 ( Interrogators, Rogue Traders, Explorers of a Rogue Trader ship, Commissars) : +150XP for career skills; +100 for non-career; +50XP for talents; max skill rank: 4

Tier 4 (Inquisitors, Space Marines) : +300XP for career skills; +200XP for non-career; +50XP for Talents; max skill rank: 5

Space Marines (there is also a Space Marine booklet in the forum post for the DH conversion, check it out) have an XP balance just below Tier 3, though with all their equipment and modified stats they'd more than make up for that. Characters besides Space Marines would be skill monkeys, basically.