Never mind, I started doing the math and it's not as bad as I thought.
Say you have an X byte file. If you download in 10,000 byte (hundreth of a megabyte) chunks from each source, then there will be X/10,000 chunks to verify. Assuming a 32 bit = 4 byte hash for each chunk, that's X/100,000*4 bytes worth of hash information. So, hash information will make up only X/10,000*4/X=.04% of the total download, which is a trivial amount, even if it has to be retrieved for each source.
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