This is what SPGsite saysNow... thinking about this one step further, you can do up to 95,000 or so SPG points in 1 day. Could you do a transfer of say 75,000 (with your 15k bonus on top of that) to make 90, thus resulting in 135,000 miles for a cost of 150k AMEX MR? Or is that limit that you can hit each day only be able to be done by 3x 20,000k transfers (meaning you'd only get the 50% Dividend Miles bonus on the first 20k+5k transfer)?
Starwood Preferred Guests can transfer up to 79,999 of their Starpoints® per transaction per program within a 24-hour period to the frequent flyer programs listed below.¹ Plus, for every 20,000 Starpoints transferred within the same transaction, Starwood will automatically add another 5,000 Starpoints!This means that up to 94,999 Starpoints can be transferred into your frequent flyer account, with most airlines, making a free flight that much closer.² Please keep in mind that only one airline transfer transaction per member, per airline program, is permitted within a 24-hour period.
So answer to your question is I believe yes you can get 135K miles to US through this (would you have the 150K in MR to start with). Max value I think is 60K transfer with 15K bonus = 112.5K US miles, in your case you get no addition SPG uplift on the 15K final points (but you do get the US Air bonus)