Remember that for CPU, your allocation is the minimum amount you could possibly get. You will be allowed to burst higher than that based on server load. You can read this article on the Linux V-Server CPU scheduler.
I think there are three important things to thing about in this area:
1. Most web services don't actually need powerhouse CPUs. You're not going to run a 3-D game on them. You're not going to factor large prime numbers either. You can't compare the needs of a web server to that of your home machine.
2. As I said above, you'll be able to burst above your guaranteed limit based on availability of CPU.
3. In some ways, you're already in this situation in shared hosting except that your guaranteed CPU is 0 MHz.
BTW, how would you quadruple your current hosting costs by choosing DreamHost PS? Are you looking at the the 10-year prepaid option for your hosting plan? That would certainly be more than tripling it as it would increase your cost by 250% - though one should factor in the time-value differences of upfront payment vs. paying monthly.
The real question is what performance characteristics your intended site has and how much traffic it's going to generate. I have DreamHost PS on a minimum plan and am fine but I mostly have low-traffic WordPress sites. Also, unless you happen to hit the server while its under load, you're likely to just see the speedy results from a fully dedicated CPU.
Have you taken a look at the DreamHost's recommendations on their DreamHost PS site?
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