We had an issue when we got our first dog at our current address. We got a letter from all our "concerned neighbours in a 100m radius". The poor puppy had just come home and was crying at night because it was unuse to its new surroundings: we were accussed of not feeding it, not loving it, and causing others to loose sleep, being the root of all evil, etc.
Anyway, turns out it was one neighbour -- do-gooder christian -- who wrote the letter, and noone else. Best thing we ever did was to find out who it was and confront them (not aggressively, but firmly). Don't try and annoy your neighbours by having mates cruise with stereos and so forth (not saying you would) because you reduce yourself to playing their petty game.
Door knock your neighbours until you found out which one/s it was and discuss the matter with them. Point out its your car (not your parents), that you obey the speed limits, that the BOV/wastegate is standard, that they may be confusing your car with others, and that writing an anonymous letter is cowardly and if they have a problem they should make it known to you in a polite and correct manner. If you feel uncomfortable with face-to-face, do as suggest and write a reply (I suggest you make it sound as "professional" as possible) and letterbox, ensuring you include a copy of the original letter to showcase how rediculous it is (both it terms of content and the lack of a name).