The problem with too small a foresight is the white you see is not true it is the light refracted over the edge of the foresight.
The eye has millions of receptor cells but there are only about 8 used to judge the gap of the white which is the precision of aim, so up to a point there is advantage in going big, this also reduces the phycological reaction of trying to over correct movement which is often much less than is actually happening. I've demonstrated to shooters with a Scatt (which is aiming path analysis using an infrared sender on the rifle) that what they percieve as massive movement is infact in side the 9 ring and if they just relax and let the shot go it will be better.
I coach, and have taken the foresight out with some shooters who have problems keeping still, with some quite spectacular results. I coach a lady who has shot 560 at 10 mts, and uses a short sight base Walther LG300 Junior with a 4.7 foresight.
Only with a very steady hold (590+) is it worth going a bit tighter on element some go down to 3.7 but we are talking people capable of those types of scores. Its interesting that the the top prone shooters (thats a very steady hold!) are using 3.7 on a smaller aiming mark.
Good shooting
Robin