Lag spikes in OBS or while watching your preview on twitch (live, not the recorded file)?
Twitch is laggy, you cant do anything about it, the only way is to lower the bitrate. If its OBS we have to see the logs. If you dont have any frame drops in OBS and your bitrate doesnt turn red while streaming, its not on your end. To check the stream quality check your recorded stream files on twitch [and local too, OBS is using the same settings for streaming & recording] - if you dont see any lags there, its not your problem.
2500 is fine for 720, if you stream 1080p - you want as much bandwith as possible to make it look good (but not too high, there are still people who have really bad internet).
I was streaming 1080p 30FPS with 3000 bandwith as a test. OBS was fine - 0 frame drops, green connection (3,9mbps upload) but the preview/channel video was buffering and i got some lags too.
Just play around with the bitrate - if 2500 looks fine for you, use it. Or if people complain about 3000+.