Douglas SBD had max speed of 255 mph and thanks to sturdy structure, the dive breaks limited the speed on 65-75 decree dive to 250 mph (source; USN training guides of the time). If it had "free dived" it could have gone faster, but it would have been too fast, reducing accuracy, making it impossible to pull out of the dive or even exeeded the structiral limits of the craft, tearing the plane apart.
As for Stuka, it was "class of its own" as allied test pilots said. The dive speed was limited by the breaks to 600 km/h (375 mph) and dive was true vertical, 90 decrees. Also, the dive was automated, meaning once the pilot pulled the "dive" lever, the plane would roll over, open the dive breaks and dive 90 decreees no matter what. Once bomb was released, it would close the breaks and pull out on automatic, with 6 g's worth. So, NO the dive breaks had nothing to do with the loss of vision in Stuka. They were there to prevent the speed to build up on dive and were closed while plane pulled up -> pull up from 375 mph dive caused the g-forces -> loss of vision or as stuka pilots said; "Seeing stars". Had the speed being more than 600 km/h (ie. without dive breaks), the pull out g's would have grown and pilot would not only had the "grey-out" but would have actually been incapacited. So stuka was really tuned to the very human limits of dive bombing.
Bottom line; Dive breaks are there to limit the dive speed during dive. Free diving would be faster, but also less accurate, more stressfull on pull out and worst case scenario; could break plane appart. Thus all dive bombers, Stuka included have max dive speeds, governed by their dive breaks.