One slip Jeff, and I am surprised both that you made it and that no one else mentioned it yet (in my quick review of the thread): you used an Nvarchar(10) to store the output. That introduces a CONVERT_IMPLICIT into the Compute Scalar operation that is unnecessary and which skews the CPU and duration of the runs.

Second, I tried this, expecting the CONVERT_IMPLICIT to float removal to make the STR() run much more efficient. I was mistaken and quite surprised that it did not make it much faster!

CREATE TABLE #t2 (a float)

GO

INSERT #t2

SELECT CAST(somenumber AS float)

FROM #MyHead

DECLARE @Bitbucket VARCHAR(10)

SELECT @Bitbucket = STR(a,10) FROM #t2