I don't understand why you wouldn't get good speed after cloning? I can imagine that a freshly installed OS does help of course, but it's certainly not bad by any means. Just the thought of having to reconfigure Mail, Parallels, Virtual Box, Picasa, find out how to backup iPhoto databases and albums, contacts, .... were for me enough reason to just clone the drive.
Be aware that if you clone a drive and replace the old drive with a new one you have to go to preferences->startup-disk and enable the newly placed disk as startup-disk. This is not done by default. What happens if you do not do this that during boot you will get a white screen for a long time (10-20secs or so) before the grey apple appears. This might be the reason some people report slow boot times. It happened to me too

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I don't think the XT is a drive which does good in synthetic benchmarks, as benchmarks tend to do a lot of random operations, both read and write. The XT shines in real-life environments, where you READ certain data a couple of times. Then the built-in software puts this data in the flash, enabling faster reads.
I guess the drive does benchmark-wise about the same as any other 7k2 disk. There are plenty of reviews out there which have run benchmarks.