Here is what is on the Porsche website:
Trademarks
Porsche, Porsche Crest, Porsche Design, 718, 911, Spyder, Carrera, Targa, Tiptronic, Porsche Speedster, Macan, Varioram, VarioCam, Boxster, Cayenne, Porsche Cayenne, Cayman, Panamera, Tequipment, "Porsche. There is no Substitute.", RS, 4S, Porsche Bike S, PCCB, PCM, PDK, and other Porsche product names, model numbers, logos, commercial symbols, trade names and slogans are trademarks and the distinctive shapes of Porsche automobiles are trade dress of Dr. Ing. h.c. F. Porsche AG ("PAG") and are protected by U.S. and international trademark laws. You are prohibited from using any of the marks appearing on this website without the express prior written consent of PAG, except as permitted by applicable laws. Other marks and logos shown on this website may be marks owned by third parties not affiliated with Porsche and are used with permission. Nothing shown on this website should be construed as granting, by implication, estoppel or otherwise, any permission, license or right to use any trademark, service mark or trade name displayed on this website without the written permission of PAG or the third party owner. The use of any Porsche logo or mark, whether registered or unregistered, as a hyperlink to this website or any other Porsche website is strictly prohibited unless consented to in writing by PAG.
So they don't own or have a Trademark on the "Speedster" name but they do of course have a Trademark on the "Porsche Speedster" name. In answer to my own question above, the "Speedster" script on the side of the car should be okay.