Been on a huge learning binge with ASP.MVC 2 lately, and recently uncovered there are different rendering engines out there... Spark especially caught my attention, couple of th
Answering the three part question in order...
It probably is a good idea to learn new things one at a time. Especially because nearly all of the MVC examples and tutorials will be in WebForms syntax. That said - it's best to learn in an experiment solution rather than your "real" project, so after you feel like you have MVC concepts down it's a good idea to create a new sandbox and try a few MVC + Spark pages.
Performance in terms of memory pressure or processor utilization probably isn't the most important consideration for all but the largest web sites... The impact on developer, and designer/creative, time may be small at first but it's cumulative and non-linear. A bit of simplification up front will save you boatloads of pain in the future, and "simple, plain syntax" is the cornerstone of Spark view engine premise.
That is very true. The polish and refinement of experience is the most expensive part of tooling and modern IDE's. I think that's why most OSS web stacks start with a great editor (cough TextMate cough) and work their way up from there. With Spark you can get csharp language intellisense, but that's clearly the low water mark for tooling support.
It's anecdotal, but one way to measure is how many people regret using Spark and switch back. I'm not sure of many - though the delay out of the gate in getting MVC 2 support out had some people wondering, I'm sure.