For many riders, the dope is a crutch, a shortcut to fitness lopping off the attention paid to diet and weight. For many riders, dope is the last item to "bridge" them to the higher levels, when all the diet, weight and controllable items have been minded. For many riders, the dope response is very great, leading many to think it is all they need, pushing them into a "crutch" reliance cycle. For others, the doping response is not so great, so they load up on more and more, exposing them to higher levels of risk in mis-managing their routines, thus scuttling their training, and thus less prepared for events than otherwise.
It is not a fitness fix-all, where a little EPO and HGH lets you train into the dark and ride past previous limits. It has plenty of downsides, too.
As to gains, a top level rider can ALWAYS improve, 1-4% on power/threshold, without dope, for peaks and targets. It is what makes training and racing so hard. Shortcutting this difficulty, so that those gains level out and become the norm, you see less delta between ceiling and base, more steady performances over time, depending on how the riders' body handles the stress.
For the big events, adding the externally stored blood is the big advantage, providing a sweeping change in capacity and recovery. What once was hard, is not as hard, and the recovery is less pronounced. Extra blood also has downsides, water weight gain, stress on overall system trying to figure out where the extra 300-800cc's came from, and starts flushing right away.
The doping is a big, big advantage, accentuated by having a precision program to maximize the upsides and minimize the risks and reactions of the body. Tuning this can take lots of money and time, more than most riders have, and certainly more knowledge than 99.9% of pros can possess. This is where the Dr.'s come in. This is Ferrari's business, Del moral, Marti, Fuentes, etc, etc.