SQL Server query disk write/sec limit
You don't say but I'm guessing that your log files are on your D: drive and what you're seeing here is writes to the transaction log files, not to the database files i.e. to ldf not to mdf.  You won't see the mdf files written until the DB hits a CHECKPOINT, which could be some minutes later.

The linear increase is good and what you'd hope for.  That you can get from 120 to 240 and 360 shows that the disk subsystem is capable of these rates.  Whether the absolute number is as good as you would hope for or not, that's another matter.  I would suggest that the nature of the test you've chosen is itself limiting the whole system throughput.  Now you have populated tables you can try

INSERT INTO TestTable
SELECT TOP 10000 * FROM TestTable;


and see what rate that gives you.