I know these days everyone is a big fan of replace, but I started getting the dreaded braking vibration so i'm wondering if anyone knows if the brembo rotors on the xjr can be cut to save some good coin right now.

They are just metal, As long and min thickness is maintained, I would think there should be no issue, but then I am just surmising.
As I understand, it is always good to surface when pads are changed in any case.

Well they were machined for the casting in the first place, so I don't see why another machining couldn't be done, provided minimum disc thickness is maintained as 'dsetter' says.

Of course, Brembo discs will be expensive, (is it mega-expensive ?), whereas for normal discs it isn't worth it with the price of new discs being so low.

There's no real reason to resurface a rotor unless it has some really ugly gouges or warping. Most cases of brake pad shudder is simply spots of brake pad material that have bonded to the rotor surface which can be removed with an angle grinder and sanding pad.

Try this before having them turned. If it works, consider getting better quality pads so that the problem doesn't come back.

Some cars have brake discs with "hardened" surfaces and should not be turned down. I do not believe this is the case with 350 Jag Brembo brakes so, yes they can be turned down.

Be aware, however, that if you must machine the disc down to the lower thickness limit to get a clean/true surface it must be replaced. You replace at the lower spec, not after it goes below that spec. To but it another way, you don't put disc brakes on the road when they reach their lower thickness limit.

I have done several pad replacements over the years without turning rotors. Twice, it has been on 2 or 4 piston caliper systems, with zero ill-effects with any of them. That's my experience.

I simply made sure that each rotor surface was smooth all the way around, on the in and outsides.

Very wise advice from Tarhealcracker.

After all, the discs will soon be below the wear limit if the freshly machined surface is at the limit. All of these discs are ventilated, and the risk is the inner wear surface, held onto the main body of the disc by cast-in ribs in the ventilation space, could break away due to the remnant metal having insufficient strength to take the brake force.

If the rotors are cross-drilled or have slots in them, then they cannot be turned down.

Otherwise, the minimum thickness is marked on the rotor hub and you can refer to that as guidance. It is customary to refer to the minimum thickness in terms of the minimum "in service" thickness. That is, can the system with a new set of pads be expected to perform properly with a rotor at the marked minimum thickness. This is because the specification was set with the expectation that the new pads will wear the rotor and additional "x" before wearing out themselves. There is no way that someone in the field can look at a specification and say that it has to be the marked specification plus some unknown padding for some unknown amount of wear in the future. In any case, any time a wear specification is put to a safety related item, it is conservative.

On the other hand, if you use the brakes gently, then perhaps you have built up deposits on the rotors that cause pulsation. It can also glaze your pads.

The fix for that problem is to perform the following on a deserted road: make 5 very firm to hard stops from 50mph down to 5mph before accelerating up to speed again. Do not come to a full stop in between. On your last stop, get off the brakes before coming to a full stop and just roll off your speed. This prevents the formation of yet more uneven deposits and uneven cooling. Let the brakes cool.

If you find that they have improved, but are still not up to your expectations, repeat the sequence.

+1 to that.

As has been said, brake pulsing is from pad deposition. Cheap or just poorly formulated pads cause the problem. When the rotors get grooved, they either need to be turned or replaced as putting new pads on grooved rotors will result in very uneven wear and low brake performance. If the rotors are drilled like the Brembos are, they can be turned down, but it takes special skill and equipment to do it and most shops will not try it. The normal brake lathe drives the bit holder screw too fast for drilled rotors, so it has to be slowed down. If the lathe is not adjustable, it won't work as well, with the bit hanging up in the edges of the holes and making a mess of the job. As expensive as the rotors are, trying to grind them may be worthwhile.