You should never buy any application on what it may do in the future, but only based on what it can do now!
If you don't like it as is, don't buy it, but wait. If it gets the features you need within a resonable timespan, buy it then or else look for something else that suits your needs better.
If you can't live without a save/load feature don't buy, but wait. different people are looking into the possibilities, so it may not be long before we do have somekind of save/load feature or and alternative for it. But on the other hand it may still take a long time before we see it, but hope not.
As for the slowdown, that all depends on what you create. If you expect to create massive worlds or structures you can explore, don't buy FPSC, it cannot do that. It can do lovely worlds and structures you can explore, but you may need to divide it over different levels.
Optimizing and creating your levels wisely you give you good speed.
Perhaps you'd care to download the demo in this link
http://files.filefront.com/landscapedemozip/;4901240;;/fileinfo.html and see how well it runs on your machine. It's an outdoor map with lot's of mountains and a number of buildings. It has been optimized to the max, so when it comes to outdoors/indoor combination it gives you a good idea what FPSC can handle. I'm still working on a indoor only demo, but that isn't finishd yet.
On an AMD Athlon 2Ghz (XP2400+), with 1Gb and ATI Radeon 9600, this level runs 28fps minimum, but most of the time above 30fps.
Imo FPSC is worth every cent/penny even without save/load features.
