One reason I would caution against rolling the dice for a handful of biglaw spots available during 3L recruiting is because the pool of available 3L biglaw spots available for you, assuming you even do get the grades to get an interview, might be even smaller than usual due to the recent economic downturn. The way 3L biglaw recruiting works is that firms try to fill slots that didn't get filled based on 2L recruiting because they gave offers to people for SA or permanent positions that didn't end up accepting the offers for whatever reason. If less offers go out and more people accept, they don't really have 3L slots to fill.
Right now, there is a lot of talk that less 2L SAs will get permanent offers both this summer and next than before. They will get no offered or cold offered b/c the credit crunch and the poor economy has hit firms hard. This is not pure conjecture either - talk to attorneys who graduated back in 2001 or so - that was not a good time to be a law student. Firms are also likely going to cut back on Fall recruiting for 2Ls. If this is happening to students at top 15-20 law schools where do you think that leaves students at a middling tier 2 or top 50 school? The point I'm trying to make is that your plan isn't well thought out and you are essentially chasing something that probably won't be there for you.
Now, I'm not saying you shouldn't transfer at all. Do it if you must.
Re: clerkships, good ones (read: state appellate and supreme or any federal) are hard to get. And you need top grades for those. Even if you knock it out of the park during 2L, you are competing against a plethora of applicants with better academic records than you. And judges are definitely going to want to see the 1L transcript for your old school.
If you want to clerk for traffic court or some other low level court that is a totally different story, obviously.