Depends on how much methane ends up in the atmosphere. A lot of methane released in a short time, could end up having very dramatic effects. For example, Siberian permafrost melting and releasing a lot of methane would probably have some measurable and dramatic, but short term effects. However, the addition of a couple of giga tons of methane released per year is not going to help slow that melting process down. If that's a process that lasts a few decades/centuries, it would raise methane levels more or less permanently throughout that period. It would recover once methane release stops/slows down.

We'd be looking at a very different planet by then with higher see levels, higher temperatures, and a lot less ice on Greenland, Antartica, and most mountains. You can haggle about just how bad things would get in terms of temperature rises, and sea level rises. But lets just say it wouldn't be very good. It would recover eventually. But the problem is that that recovery process is going to take thousands of years.