That is correct; I've just done the math for it and confirmed it.
Profit for star cut for a $400 gem would be:
You send off $400 for the gem, $400 for the cut
You get back a gem worth $400*4 = 1600
Total profit, $1200
Profit for a heart cut for the same gem:
Send off $400 gem, $1000 cut
Get back a gem worth $400*5=2000
Total profit, $1000
The star cut is most profitable for gems worth $300-599.
So the updated list of best cuts according to original gem value would be:
0-40: Don't Cut; sell as-is
41-60: Oval
61-83: Plumbbob
84-171: Crystal Ball
172-299: Brilliant
300-599: Star
600+: Heart