From: Vancouver, BC
I just completed a game against the AI to test out an East-first opening for the Central Powers. That worked very successfully: the Bolsheviks seized power in late September 1915 and offered to withdraw Russia from the war. So, I decided to test out what would happen if I declined to sign the Treaty of Brest-Litovsk.
Russian national morale immediately rose from 8% to 39% with the infusion of 15,000 NM points. But the Central Powers still had a huge advantage in manpower, generalship and unit morale advantage over the Russians. So, I pressed forward towards Moscow and Petrograd. The going was slower going than before because the vast spaces in Russia made it harder to bring my artillery to bear on Russian corps entrenched in towns along the way. But I offset the reduced tempo of land combat by sending the German fleet into the Gulf of Finland to slaughter the Russian navy, which had been kicked out of all the ports in now independent Finland.
By early 1916, I was beginning to doubt the wisdom of this strategy as successive waves of defections struck the Central Powers units in Russia, due to Bolshevik agitation. It must have happened at least three times and affected 10-15 units each time, each losing 1-2 strength points. Nevertheless, I was able to capture Moscow and then was just able to capture Petrograd, just before Russia surrendered due to their national morale dropping to zero.
It was like winning the jackpot in a casino. The Germans plundered an eye-popping 2650 MPPs from Russia. Plus their economy shot up from generating 700 MPPs per turn to about 1150 per turn. There were no break-away republics - all of Russia became CP territory. And while the usual Bolshevik agitation decision-events continued to slightly affect German and Austro-Hungarian national morale for the rest of the game, the defections from the CP armies stopped. Plus the UK, France and Italy all took a bit hit to their national morale.
I used this colossal windfall to build every capital ship that Germany could purchase (5 dreadnoughts and 3 battle cruisers) and used them to completely destroy the Royal Navy the following year. With the German economy producing 1150 MPPs a turn, the research engine could run at maximum capacity every turn in all three Central Powers. It was as if the MPP cost of any move ceased to be constraint on decision-making.
It must be said that I was playing against the AI on the intermediate setting, so I could afford to take the 10 extra turns to completely defeat Russia without worrying about a destabilizing counter-attack from the Entente. A human opponent would have found ways of making me regret not having the units tied up in Russia available to shore up other fronts. But based on this experience, the pay-off for refusing Brest-Litovsk is so huge I would certainly consider trying it in a MP game. I will be interested to hear if anyone else experiments with this option.