The incompatibilities Illustrate the need for FFG to perhaps set things up with a living rulebook [far as I know, barring possibly old battletech, it would be the only sci-fi system with one] to keep things updated.
There's a constant crossover of equipment and adversaries by groups and GMs, though a party will generally be built all within one system. Their XP values aside, it certainly won't change the fact that, as I tried to get across in my example, its very unlikely the stats will be all that different. And you know what? That's overall a good thing. The rules have greatly improved, have gotten quite refined, and though they certainly aren't ideal everywhere they deserve a unifying 'reprint' to bring the systems closer together by this point. It also means we can estimate what folks'll be dealing with a little better.
A Chaos marine will certainly have at least 40% agility, TB8 and be wearing Power Armor. Its highly unlikely his Legion Bolter will have dropped in power or his wounds be below my rather conservative 16 either. In fact, the NPCs are usually roughly equivalent to a character that's purchased a single advance in four or five characteristics but otherwise rolled quite average. The Chaos example above was running on very IG-Friendly, fatally optimistic numbers most likely.
Bloodletters for example have only changed in that their movement rate in OW has the modification from their Hulking size written in, rather than just the Agility Bonus typo'd in Black Crusade. NPC Guardsmen are the same in both games, Kabalites seem to have lost dark sight at first glance but are otherwise identical, and the same goes for all those weapons. Further examples will, with a strong majority, give similar results, but would require digging up through multiple books of each game since there's usually different examples [one book has mandrakes, the other grotesques… almost as if the designers knew we'd probably own both and don't want the same critter six times in a row]
Additionally, if you wish to compare starting XP values, you also have to compare the XP systems. Those 13000 in Deathwatch, when the difference is given to a Dark Heresy, Rogue Trader or Black Crusade character, cause the latters to catch up and surpass the marine's capabilities in most fields quite handily; and paying almost triple anybody else's costs, the difference will just continue growing. Rogue Trader had a similar problem with imported Dark Heresy characters of course.
Ideally, all of those would be unified under a single XP system, the costs and values recalculated as they've been obsolete for a while now, and from that, equivalencies listed out.
Oh. Also, you confused the "if importing into this system" 'total values' with the extra characters start with at creation. 600 or 300 is NOT the total value of those characters.