Hi John,
If you add bigger brakes you'll be on ABS quicker which will make you want bigger tires. (dirty little upgrade cycle no? j/k) Anyhow, your best bet is to get some more aggressive rubber on the car and try things from there. The Sportline suspension should really be able to take advantage of the grippier rubber. The stock 300E brakes are pretty stout so they should be up to the task of street driving. If you were going to do some heavy duty racing then the brakes might be wise, but for 99%+ of the time you'll get more benefit from the tires/wheels.

That said, Monoblocks may not be the best option for a M103 motor'd car. You'll actually get better performance, ride, braking, etc by getting the lightest wheel that provides the best brake cooling. (keep that unsprung weight down) Talk to Luke from Tirerack (wheels/tires forum) for specific recommendations there.

Hope this helps...Lee