Last year i was donating plasma twice a week while doing marathon training. It was my first marathon and i was only hitting 40-45 miles a week. At the time, i didnt notice any ill effects. I made sure i was hydrated and eating healthy, and it seemed to be fine. I was tired all the time, and did feel like i lacked energy during workouts at times, but i attributed it to training for a marathon.
Now i'm training for another one at a much higher level. After being told over the summer by my superiors at the running store i work at that it decreases blood volume (which i didnt even think about... yeah, retarded), i realized it wasnt a good option. Havnet donated at all this semester, and i'm feeling about 100x better. Putting in more work, faster workouts, more mileage, and i'm not tired all the time. My times are significantly better.
If you're serious about your running, at least for right now, dont do it.