You are correct, but you forgot a factor: magnet strength. A 3-pickup LP with A2 magnets will probably have a similar amount of sustain to a 2-pickup LP with A5s (that is otherwise identical). I think so atleast. Also, adding an A2 magnet pickup won't hurt sustain as much as an A5 or ceramic pickup.
For what its worth, I have a 3-pickup SG, and thinking about it the sustain may be very subtly less than my Les Paul from before. If it is though, its hard to tell- I really can't make up my mind either way, given the different construction and materials (the Les Paul was Alder or Poplar!) My present amplification has much less gain now too, so I think really the difference is negligable.
When I've had a problem with pickups being too close to the strings it has been so severe as to give a strange modulation type behaviour to the sound, without reducing sustain enough to hinder playing, so I reckon you'll be alright.
'My Sound? Hell, if I can hear myself, that's my sound!' -Carl Perkins
You're about to spend as much money to buy a guitar as most people spend for the down payment to their first home, and you're worried about sustain?!?! If'n you want beaucoup sustain, shouldn't you be fondling a PRS or two? After all, Mr. 128-beats-per-note (Carlos, who else?) switched from LP to PRS, didn't he?
JohnH has done some experiments, and he has found that while the negative effects of a magnet can be noticible, they are nearly always greatly offset by the positive qualities. For more info (on this forum), search on "Dreaded Tone Suck".
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