I'm not 100% sure, but I tested it onee time because i was wondering the same thing. I was a 50 siren at the time, and I was testing it with a defender class mod. I went and respeced so I had all my talent points back and then i put my defender mod on and my shields increased by a lot. It had +42% shield capacity and +3 diva skill. Then I put 1 point into diva in my talents and my total shields increased by like 150 or so. And then I put another point in and it won't up anoter 150 or so. And then the next point I put into it only increase my shields by like 10 or 20. So you can get over 5 points in a skill but it does a very reduced effect. From this, I respeced and tend to not put over a total of 5 points (mod and talents) into any skills. Hope that helps.