The XO rim the O.P. posted about is 32 lb.
B8 RS5 5-arm rotors are ~14.1 kg, which is ~31 lb.
While I can appreciate concern over additional unsprung mass when one is performing at the 10/10ths -- and they're certainly not lightweight by today's standard -- they are arguably far from being "one of the heaviest wheels ever put on a late-model n/a sports car". Although this description also turns heavily (pun intended) on what one considers "late-model" and "sports car". ;)
High-quality (H&R, Eibach, etc.) hub-centric spacers, properly installed, are not a problem.
High-end Porsches track with them all day long. Porsche would not sell 5-30mm+ spacers for 911s on their own website if there was a problem.
Effects, if any, due to the additional mass of a high-quality spacer centered close to the axle are negligible.
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