The E-Type’s brakes were considered the best out there in the day. They are better than good for normal use.
Modern brake boosting systems give modern cars a much lighter pedal pressure so some think the brakes are bad because you have to push quite a bit harder. That’s not an issue with actual braking, and of course it gives you a wide band of force with which to modulate them. Just push harder (man up)! Green Stuff pads (possibly some others) really transform brake effectiveness, too, be sure to get them. Some like Red Stuff but I don’t know about them.
Yes, S1 brakes are regularly sleeved to prevent corrosion, and this works very well. White Post is a good place who sleeved my system.
That said there are some specific issues with 3.8 brakes – and all S1 brakes. Their location inside the suspension and above the exhaust makes the rear brakes subject to brake fluid boil and loss of braking – but this occurs under really extreme conditions. I encountered it (stock 3.8 brake system) after 20 minutes of hard laps on Laguna Seca, which is a hilly track with lots of high-speed braking. I encountered it – once only – after an hour of crazy driving on California 36 (“Warning – twisty road next 140 miles”). You’ll never encounter it on the street, and you’ll never encounter it at all if you flush and refresh brake fluid every 24 months as required and use high-temp brake fluid (like Castrol DOT4) – brake fluid absorbs water from the air and this greatly lowers boiling point over time.
Another rarely-encountered issue is braking at interstate speeds in very rainy weather after a long stretch without braking. The system cools down and water on the disks takes a few msecs to scrape off. The result is that when you push the brakes there is a short period when nothing happens! Never had this happen, but of course I never drive the car in heavy rain any more. If you do, you just need to remember to brake lightly every few minutes, I guess.
If you do change the system, you are likely to reduce the overall braking at the max unless you carefully rebalance the size of the rear calipers with the fronts. Consider: braking hard, the rears may be providing less force than the fronts, and the fronts may then slide before the rears reach their max. The total braking force being applied just before the fronts slide will be less than the force provided by the stock brakes in good condition.
Note that with the stock system you can get the same imbalance and reduced braking if you use different brake pads front and rear. Very noticeable!
Note that the braking force applied just before sliding is going to be limited by the tires not the brakes, and will be never more than what the stock brakes provide – bigger brakes don’t provide more braking force.
Bigger brakes can provide better modulation, better resistance to heat and fluid boil, lighter pedal pressure. Also, cheaper cost of parts, maybe.
Also note that the biggest issue at max braking is not the front brakes but boiling at the rears, replacing the fronts won’t change the loss of brakes at extremes.
Psychological advantages to having “improved” the car, even if you actually diminished it.