my guts feelings says that the only way to get the benefit of those bonus sr boost is from solo. because when duo/trio/quad que you are effectively skewing the avg sr of your team to pit against similar sr opponent. but in solo que, the game can throw you into a higher sr games where you are one of the lower sr which benefit the most from losing less, winning more sr
but during those 10 placement match, duo/trio/etc que doesnt effect to much of a skew to the SR i think. so best just duo/trio for placement, afterward solo que until you find the sr bonus is getting taper, then go back to duo/trio que = profit???