Quoted: It seems strange that the FSB is technically obstructing the view but it apparently doesn't distort the image. Probably some obscure optical mathematics at work.Actually, it's pretty straightforward. The FSB is close to to the topic that when you magnify it, it blurs out into nothinginess. This is also referred to as "ghosting". I've experienced this on magnification as low as 1.25XThink those rings are high enough? The LaRue Tactical SPR-E mount?
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As long as the optic will clear your BUIS and not touch anything else on the rifle then yes, they will be high enough. Remember that the lower the optic is, the better. A lower optic will allow you to get a solid, consistent cheekweld and, since it will be closer to the bore, it will be easier to to hit targets precisely at distances less than your zero with minimum "Kentucky windage".
Since you're going to buy the optic and mount from Grant, shoot him an e-mail and tell him which BUIS you have. The odds are very good that the set-up you're looking at will work perfectly with your rifle. The extended eye relief of that optic actually makes the height of your BUIS less critical because the eyepiece of the optic is in front of and not over your BUIS. I just received an Accupoint TR-21 w/SPR-E from Grant a couple of weeks ago and, because of the extended eye relief, the optic's eyepiece is way in front of my BUIS.
ETA - The Leupold MR/T you're looking at (the one in the thread you linked to) is a 1.5x5x20, not a 3x9 as you guessed at in your post. I just wanted to make sure you were clear on that.