Brusilov in charge in 1914?! I have to admit, this made me squee a bit (I love being a nerd). How far do you think the Russians could advance into East Prussia under his command anyway?
Based upon logistics limitations, and the german withdrawal plans etc. I see a Russian offensive that takes the whole thing as pretty plausible before they have to stop at the vistula. The fun part is what happens then. OTL the Russian plan had 4 of their 6 starting armies against the AH.
The plan was to absorb the Austrian attack they knew was coming and do a kind of WW1 backhand blow on them. Conrad duly attacked, and got himself punched in the face. Minus Brusilov the Russian counter will plausibly be less effective, but still drive them back to the Carpathians.
OTL the Russian mobilization was completed just about the time the newly formed 10th and 11th( i think) armies needed to be fed into the NW to shore up the wreck of Tannenberg. ITTL the NW front is victorious, the AH is still driven back, and just about the time the Marne is starting the Russians now have 4 armies on the Vistula and southward, facing whatever is left of the German 8th. And there is a really big gap between the Germans and the Austrians right at Silesia.
Given the all or nothing brilliant strategic planning of the Germans at the time, i can easily see them determining to "follow the operational plan", in the west, and still figuring to turn east after France is defeated, lose the Marne, lose East Prussia, have the AH defeated, and lose Silesia. Then they have to send half their western armies to the east to shore things up, and lose the race to the sea and have the west go pear shaped too as the are driven back into Belgium.
And then they run out of nitrates.