fades but I've read somewhere that early alloy calipers (which mine has) suffers from flexing.

>> Ian, brake fade and caliper flex are two different things. The fade you're encountering might be caused by not enough air circulation. Removing the backing plate/splash guards on the front brakes helps a bunch.
>> The alloy caliper flexing is allot more subtle ... usually you only notice the difference after you change to steel or upgraded brakes. In a light car there are still many who swear by the alloy brakes and have the class records to prove it.
I found that changing over to steel brakes was a pedal feel/confidence builder.

I don't want to spend too much on a upgrade, about $1000 max.

>> Then you can’t get to crazy

My stock discs all-round are in very good condition. If I put SC calipers at front, do I leave the rears alone as it is? Would the performance substantially improve? Should the brake lines be upgraded as well?

>> You would not have to change your rotors (discs) and you would not have to change your rear brakes. Steel lines are a good idea (getting the correct dash # ... plenty of tech articles about getting the right brake lines) I feel, with the appropriate pads, this set up would be an improvement.

How about 930 calipers/discs? Did 930 cars have booster or not?

>> For the first 2 years the 930's had the same alloy brakes you have. In 77' they added a booster, and from 78' on then ran 16" wheels and the BIG brakes.

I didn't know Boxster set-up could fit inside 15" wheels?

>> Dave at TRE sells a conversion set up ... it will definitely bust your $1000 budget. http://www.tremotorsports.com/

>> Hope this helps