Try Everest bandwidth benchmark to see the difference between both setup.
however, pushing the ram alone itself, won't yield much increase in performance.
if temp./ocability allows, try 8x500 1:1 should be a sweet spot.

& i'm curious wondering how u got ur 3gb ram runs at dual channel symmetric mode?

The possibility is that the bandwidth of 1000mhz C5 (4:5) is higher than 800mhz C4 (1:1)
that's why will be good if test it with everest bandwidth benchmark to confirm it.
@ the very same fsb for both setup, it's either the extra mhz made the difference or performance level (trd) does it.
i'd say if it's the mhz itself, then the difference is minimal due to looser timing as well.

as for my curiosity, nvm bout it
it might be cpu-z don't know display async. mode.

u mean ur 800-c4 passed ibt but not the 1000-c5?

might be not enough vdimm for that,
or, try 5-5-5-15