All of which are best used on scratch disks. An external 7200RPM drive attached via FireWire will give you WAY better value with no appreciable loss in performance. SSD isn't going to do you any good for scratch disk activities. Trust me, I know exactly what you're doing with what you're doing, I do the same thing with my 27" iMac. All of the scratch files go to an external 7200RPM Western Digital FireWire800 drive. I can encode videos and songs three times faster on the iMac with its platter drive than I can with my MacBook Pro and its SSD. Besides, all of that file activity will just degrade the SSD faster.
No, no it does not. Your processor will do way more for previewing samples than a SSD. Your problem is that you're using the wrong machine for the job. The MacBook Pro i7 can get it done with a 7200RPM drive; the i7 iMac will do it faster with the same drive.
Your problem is that you're making assumptions. I've been using four different SSDs over the last two years. There are three primary benefits: boot speed (read), application speed (read), and virtual machines (Read/write). Your use case is scratch disks. You won't see an appreciable benefit for a SSD vs. a platter drive. An externally (Firewire 800) connected 7200RPM drive, or even if you want to go to the 10kRPM drives - the Barracuda, I believe - or the Momentus XT, will give you the same or better results at a reasonable cost point. Scratch disking is the worst use case of a SSD I can think of.