Sig I won't deny that eldar have better tech, but better tech doesn't mean you instantly become masters of tactics or warfare, it just means you have better tools. That's something I don't think anyone would argue Eldar don't already have. Although from the fluff we have from Armada, marine vessels seem to be almost entirely automated with the chapter serfs on board in a monitoring and maintenance function. While inferior to eldar tech, it seems a far cry for the IN's massive use of manpower to operate vessels.
If you are so sure that eldar shouldn't be in the same league, then why not adjust their LD rather than the marines? Just give them 10's across the board and you're done since they've had thousands of years of practice compared to humanity. Of course, we can't do that now can we because it would make Eldar too good. See, the thing is just growing up on a ship doesn't translate into tactical capability. It translates into technical capability in the form of an intimate understanding of how the systems on the ship work, but it won't make you a tactician. Knowing what your ship can do doesn't equal knowing when you should do it. That's where training for combat and leading others comes into play which is what the marines do a good chunk of their lives when they aren't actually fighting.
Now you'll probably want to come back and wave your hand with some statement about how dare I not agree with you, but I do not agree with what you presented. It makes a case for eldar having better tech (which they already do) and better knowledge of celestial phenomena (which they do already here too) but it's not a valid argument for dropping Marine leadership to AdMech or IN levels.
Ray, I realize they aren't crewed by Marines entirely. Marines get the best resources and the ships are commanded by the marines, who are some of the best leaders available to humanity. If AdMech get refits, then at the very least Marines should use the Admech LD table with the option to take marine captains inexpensively for +1ld.
The way I see it, IN should have the worst human LD, followed by AdMech being slightly better, followed by Marines at the top.