The Turbostar 540+ is in effect a Turbostar 432 running at 5.53 MHz, rather than 4 MHz, with the Kasparov Selected Openings module built in. The only way of distinguishing a 540+ from a 432 is thought to be by the white labels added to the back of the chess computer and to the front of the box. So the 540+ could more accurately be described as a dealer upgrade rather than the separate model suggested by the SciSys advertisement below.
In the 1986 Fall/Winter edition of the American publication Chess Computer Reports (Pages 79-81) it was reported that SciSys would be developing further models of the Turbostar range, to be known as the 640 and 740 running at 6 MHz and 7 MHz. So far I have seen no evidence that these models were produced and released for sale.