Should have said “in practice may used like a register page” as I saw a lot of work are based on this practice.
For the “not much difference” would it be intended because it is using space to trade of low speed. Given this page use less space and less speed, would it still be important.
For SWEET16 the choice to place it's registers in zero page, or not, would only affect speed of access, not code size. The SWEET16 code would still be using one byte register instructions in either case.
As far as speed, given that the interpreter overhead of SWEET16 means it's about 10x slower than 6502 code, adding an extra clock cycle for non-ZP access would at worst make that 11x vs 10x. Would that really make a difference to the cases where it was useful? The trade off is that NOT using ZP for SWEET16 would give those 32 bytes of ZP (16 registered x 2 bytes) back to the developer.
However, It seems that SWEET16 was not much used, and I doubt that any change like this - where to place it's registers - would have made any difference to that. It's maybe interesting to consider would a different interpreted code (with different feature set from SWEET16) have been any more widely used?
Yes, you can consider ZP as a page of registers, but it's not like there was any choice - seeing as the 6502 basically had no registers (just A, X, Y) you had to have your program variables in memory, and ZP was faster/smaller that the alternative!
For the “not much difference” would it be intended because it is using space to trade of low speed. Given this page use less space and less speed, would it still be important.