Well this is confusing. The client update hasn't rolled out to me on steam yet (the last modified date of the RocketLeague -binary is 31st of January, so about 2 weeks old), but by updating to a more recent mesa (where the buffer fix is inplace) the game's lag spikes have been definitely fixed. Also, the lag spikes were gone only after updating the drivers. It can't be the client, so it must be the drivers, right?