Please forgive the noobish question, but given the Wilde P46 pickup is good for 4, 5 and 6 string basses, assume string spacing is not an issue if I want to install on my 18mm 5 stringer?
Should be just fine. Those things act like bar magnets so spacing is good. Are your pickup routes standard P-Bass size or one long and one short. It'll still work, you'll have an open space where the long one is routed. Also, the A string will be picked up by both pickups, giving it a wider sensing length. Tagging @Ric5 as he may be able to elaborate if that makes a big difference or not as he has done a lot of that configuration. good luck.
I'm not @Ric5, but I have a little bit of experience with this. I had the P46 (and J45) in a Warmoth deluxe 5 string whose spacing was 18mm, and the routing was for a regular 4-string P pickup. The pole pieces seemed to cover all the strings well, and doubled up on the A, as mentioned by @Datsgor. I thought I noticed the A string being a touch louder than the others. I eventually changed to a different PJ5 set where only one of the P pickups had poles under the A. I *think* it made a difference, but it could also just be post-purchase rationalization. To be honest, other than *feeling* a difference in the A string, I don't think my new set was an improvement over the Wilde. Unfortunately I don't have any recordings of the P46, but I did have it on that bass for at least 4 years. I'm keeping them in case I end up with a PJ 4-string at some point in the future.
Me as well. I had a pair of P46 in a five-string. It was subtle, but I could tell that the A string had a thicker tone than the others. I moved them to an eight-string, and put EMG P5s in the fiver. Both sound amazing now!
A bit different to your thinking; maybe have a look at the Nordstrand Big Blades as a possible alternative? I believe string spacing is basically irrelevant.
P46 in a fiver for me also. The A string sounds slightly different as both pu halves sit under one string. Will be changing it out. Just been lazy and not Playing 5 much. the sound of th pu was great. Just not even. Saving it for a 4 string. Was totally bummed.