Here is my two cents...
Firstly, I have two MIJ jags. I went and got the local techie to add shielding and it cost me a fair bit and did zero to the hum. Then, when I bought some Seymour Duncan replacements the hum pretty much disappeared (I still get a tiny bit of noise, but not more than you'd expect from any single-coil guitar such as a telecaster or stratocaster).
I now have US jag pickusp in this guitar and it rocks without much hum at all.
So my experience is that te hum and noise is more to do with the cheaper Japanese electronics and pickups than it is to do with lack of sheilding in MIJ jags.
Secondly, the single-coil-sized humbuckers you have mentioned (hot rails and Jeff Beck Jnr) will do just that - they will buck hum - as in significantly reduce the noise and hum. However, both of these pickups - and every single-coil-sized humbucker I have tried thus far are all very high output. This means that you wont get a vintage sound out of them, like a Gibson burstbucker or anything like that, you will get a high output screamy bitey sound that will sound very 80s and be at home in Joe Satriani's screamy Ibanez guitars. Every note will scream and if that is what yoiur after (that 80s lead sound), its fine, but it aint a vintage tone by any stretch of the imagination.
Best option in my opinion is to replace the pickups with some good quality/well made single-coils. All single-coil pickups do hum a bit, but the better quality ones do it a loss let than the standards in the MIJ jags and single coils are so much better in my opinion, clearer, subtler and better range of tones. Certainly for surf, psychedelic or a sound similar to Interpol (my current sound) singlecoils on a jag are better.
Try Seymour Duncans (but not sjag1s), Fender US standard Jag, or the replacements that I highly recommend from Vintage Vibe (pete rocks and his p90 type jag replacements are the best pickups ive ever heard in a jag! Ive had them for only two days and I havent left my second jag alone - infact Im comtemplating naming it number 1 instead of number 2!)
lastly, If your playing surf style music, then you'll want the tremolo to work. This means you need wither a jag bridge or a mustang bridge, which are both made to rock with the tremolo. If you replace with a tune-a-matic the strings will stick and detune/break. I dont mind my jag bridge once its set up right. Your best bet is to spend the $60 and go get it set up at the store if its a stubborn one. My first was fine, the second was a pain in the arse and in the end I had to replace it with anohte rjag bridge. Worth it though. Looks very retro.
