The verification speed when verifying blocks past the latest checkpoint is far slower than getting the blocks from the network usually.
Experimentation being the basis of science I tested your theory. I used a AWS m3.xlarge with a 60GB SSD block device for the bitcoin client. I created a bootstrap.dat for block 305000 and compared the time it takes to update from block 295000 to 305000 via my bootstrap.dat and by pulling blocks from the live network. It turns out you are right.
Time to update from 295000 to 305000 with a custom bootstrap.dat: 3845 seconds
Time to update from 295000 to 305000 by connecting to the live network: 3848 seconds
As a 3 second difference on a 1 hour load is meaningless I can only conclude there is no reason to update this torrent. I'll keep throwing bandwidth at it.
One flaw in your test: I'm pretty sure when you give the Bitcoin client a bootstrap.dat file it doesn't just "update" but instead rescans the entire blockchain database on your hard drive. That's why the bootstrap.dat is meant to be used on a fresh install because if you already have a good chunk of the blockchain downloaded you're usually better off just letting the client update from the network.
Also not everyone has the bandwidth of an extra large AWS instance. With my slow 3mbps/384kbps DSL line, I may be the exception in that I could possibly rebuild my local database faster than I could download the latest blocks.