Hi guys, so I've had a look around on various forums for information on this and it turns out there isn't much so I thought I'd post my own from my current experience.

I have a strat, and the guy I bought it off had got a custom brass (I think) pickguard for it. It looks absolutely killer, but I've found a few pros and cons that people who are thinking of doing anything similar should probably be aware of.

Firstly and most importantly, microphonics. If the pickguard is made of ANYTHING which contains even the slightest amount of magnetic material (iron, nickel or cobalt most commonly) then your pickups WILL receive a signal from it. If your pickups are also even remotely microphonic then this will be an issue. I had to completely re-pot mine to get the issue to calm down. It's also worth using something to try and damp the vibrations of the pickguard. I used a combination of foam (the stuff you get in the bottom of new pickup boxes), and a thin layer of rubbery electrical tape. Coat the whole bottom of the guard in a layer of tape (no overlapping if you can), and then cut up the foam and push it in around your pickups and in any other large gaps you can find (the more you fill it, the less microphonic it is)

I haven't noticed a HUGE difference in tone (possibly just a touch brighter? Very hard to tell), although I feel like there is a good amount of extra sustain. The sustain was reduced back down to normal once I filled it with all that foam, so I tried to go for the best foam:feedback ratio to keep both at their best!

The only final thing to add is that you absolutely MUST ground them or the hum is absolutely unbearable. My ground wire came loose at one point and it is INSTANTLY noticeable, even on a relatively low gain setting.

All of this was done and experimented with using some pretty high gain metal settings with a Seymour Duncan SSL3 in the bridge, and the original Vintage Series American Strat pickups in middle and neck (all three pickups had microphonic squeal when I started out, so don't assume that if your pickups are low output you'll be fine..)

I hope that someone finds this information useful, and if anyone find any better methods or additional info then let me know and I'll add it onto the list
Personally, I have gone with metal guards on and off throughout the years and if your guitar is properly shielded and everything that needs to be grounded is grounded then everything should be peachy. The vibrations don't really bug me so I don't bother with it but I have seen other people do similar things that you do with the tape/foam combo.

I have never really noticed a difference in tone from changing the guard though. YMMV
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I stick (grounded) aluminium foil to the back of my pickguards as shielding, and I can't say I've noticed any effect except reduced rf noise. The idea of using a ferromagnetic material surrounding the pickups is intriguing, Some say this contributes to the tone of tele bridge pickups and the old Valco string-throughs, for example. It might be worth experimenting with - microphonic effects can be a plus in some circumstances.
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It might be worth experimenting with - microphonic effects can be a plus in some circumstances.

I don't actually have a strat with a pickguard (my strat-alikes don't have pickguards), but I'll file the TS' post for future consideration.
Meanwhile, I actually prefer non-potted pickups unless they just get out of hand. Gibson pickups, as they come on their guitars, are almost always potted. But if you buy them aftermarket, they're often not, and I sorta like them that way. I have a couple of guitars that are a bit "loose" in a microphonic way, and if you're in a high-volume atmosphere, they can be fun (or, of course, a complete nuisance). On several of my guitars, I have sustainers installed so that I can imitate that feedback loop without the tinnitus concerns.
Some magnetic soundhole pickups, eg the Baggs M1, are made to be highly microphonic, so that they pick up body sounds and "acousticness". I think it is possible that this "acousticness", could be an asset in an electric if you are looking for a good clean sound.

A ferromagnetic pickguard, or at least part-pickguard would be easy enough to arrange - an old bean can.