When I recently moved guild I moved up a fair number of steps in the "boss kill" hierarchy, moving from 1/5 in Mount Hyjal, and 0/9 in Black Temple to regularly farming 4/5 and 4/9 respectively, with the kill on Gurtogg Bloodboil a week after I joined (and I participated as one of the three tanks), yet I didn't feel boosted through the content, I tanked a large proportion of it in that week and put out similar threat and avoidance to the other tanks, my shortcut sped up my progress through the game, but was at the gear and experience level I had. The question is though, if I had applied to a guild that had finished Black Temple, and was looking to gear up a tank for the Sunwell Plateau would I have felt boosted, and would this really be a bad thing.
In our guild we have a new recruit (well a week newer than I am), and he has no real 25 man raiding experience, but a lot of Karazhan experience. He worries that he won't perform and won't pull his weight in these instances, yet this never worried me for him, he knows his class, performs well and already has a high proportion of his potential damage at his gear level, in short the only thing he is lacking is having seen the bosses and played with 24 other people at once. Key to my not worrying though is that in a 25 man instance each person generally pulls less weight than in a 10 man instance, ok there are exceptions, Teron Gorefiend (1 person failing can wipe the raid with constructs), Archimonde (Dying causes massive damage to everyone else), Brutallus (29,000 raid dps please, not easy on progression), but in general you are focused on your part of the raid, your small section and your movement, the rest of the raid kindof just does its thing and provided each part does it it comes off. Taking the Shade of Akama as an example there are 2 tank groups whose sole job for the first part of the fight is to hold adds and ideally kill them while the other group takes down the channelers, in the second part the tanks and healers have to survive for 60s while the dps nuke the boss (threat free fight). I haven't really seen this fight, I stand to the side and try not to die as my adds pile up, the main body of the work doesn't concern me provided my 3-6 man group can keep me standing for the duration and I don't let anything through.
In contrast in 10 man instances everyone counts, you will often have fewer healers per tank, fewer dps, and a lot less synergy, you can't afford to slack and let people relax because there is no backup, any death can really push the group, a tuned pull such as in Zul'aman requires you to have your CC up, or be capable of dealing with massive burst damage. Your environment is closer and the challenges different, you work as one small group, or several micro-groups, in effect fulfilling one or two parts of what a 25 man raid is capable of, but doing so all the time, there isn't a "trash-slack" part where healer mana isn't an issue, nor is there a general tank-slack, I can't afford to simply expect one of the other tanks to pick up a broken enemy if I am AoEing, nor can I expect a feral kitty to pop tank and pull it back for us, the resources are a lot more finite and the awareness has to be higher. So our guy with no experience is doing the same, he is watching out for others, preparing to trap, watching dps and threat, in a 25 man I wouldn't really expect his to trap at all, in general zerging or Mages handles it, or there are spare tanks to deal with issues, he does more and has to be willing to do more in a 10 man instance than in a 25 man.
As a side note I was right, he came into Gruul and Magtheridon, ok with an experience and overpowered group, but that doesn't help him, got a few terse explanations and performed well, his knowledge and movement were fine, because he applied the experience he had from previous raiding levels, he kept his head and wasn't worried about the big picture, he did his job, and thats all you can ask and expect from a raider, because in any level of raiding we can cover some slack, but fundamentally its a 10 or 25 man instance for a reason.
So whats the point, really I don't know if jumping content is a good thing, but with a sufficiently aware player I don't think its a bad thing, you don't learn a lot more dodging Leotheras's whirlwind than you did on Hydross the Unstable, the aggro wipe mechanic is the same. You need to know a certain amount and have the skills to progress, but when you have them you only miss out on the learning portion of a boss, the sweet first kill, in fact there is probably no such thing in the end game as being "boosted", because everyone needs to actually fulfil their part. Your individual weight and contributions might be lower, and your death might not be a wipe, but it sure won't let your raid complete the kill easily, requiring on the fly group changes or massive overhealing and dps pushing the envelope to do it. In the end, hit the ground running, because the treadmill doesn't stop.