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Even a stock 135i with street tires will need attention to brakes for track day use. You can get a lot of pad fade if using OE brakes, so you need to watch out for that and drive with adequate margin for safety. OE pads are not designed for high temperatures experienced in track day running. The stock pads may be usable if you are cautious, but personally I wouldn't risk it.
When you put R-comp tires on a 135i that is also fitted with engine cooling mods for track running you will place very high demands on the brakes. IMO the biggest failing of the stock brakes is the limited cooling capacity of the stock rotors. From what I have seen, even the 335i single piston brake setup has superior cooling to the slim 26mm front discs that are stock on 135i. The brake cooling may not be as bad on high speed tracks, but I found the lower speed tracks generally don't allow sufficient cooling and are more likely to overheat the brake pads and caliper. Without brake ducting you will get very high brake pad temperatures in this situation so even moderately race/rally style use pads will be pushed to their limits on the front of a 135i.
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