I’ve not been in another P6, but it used to detract from the enjoyment of the car. Now it feels less intrusive, and is on par with the engine/transmission noise and wind noise. I don’t expect it to be as good as a modern car but given the Rover’s reputation for ride and comfort I expected it to be better than it was.
I did quite a lot of work in this area since my post back in 2015.
Cheap checks - make sure the suspension mounting bolts are correctly torqued (and the lock tabs are in place).
Is the interior "airtight"? i.e. make sure any rubber grommets are not missing. As stated the missing rubber cone on my car on part of the accelerator linkage let in far more noise from the road and transmission than you'd ever think possible through such a small hole.
Simple questions. Is all the original sound control in place? There are thick jute mats pretty much everywhere. If these are missing or disintegrated, then it's actually better to use modern stuff (jute isn't dense enough especially to block lower frequencies - I used a roll of Techsound 50, cost about £50 and you could do 2 cars with it - so share the cost with someone else.
Similarly on the firewall there are two sound control mats. These are thick hardboard and dense foam. Mine were missing when I got the car - now replaced.
The bitumen pads for damping - this is on the firewall and transmission tunnel . The bitumen pads dry out and go brittle and peel off - if they aren't in contact with metal, then they don't do anything. You can replace these with the modern stuff (ie dynamat), it works better. You must clean the surfaces with solvent and use the proper metal roller for best results - as stated damping only works by contact. It's actually quite a bit of work to do properly. I've seen this done badly many, many times.
It might be best to pay attention where Rover did. Clearly they had issue with noise coming from either the rear suspension or the boot area because the under seat area seems to have far more sound control than any car of that era I've seen. Exhausts in older cars aren't as well mounted and an empty boot will act as a nice resonance chamber. Fill the boot with something dense and go for a drive and see if it makes a difference - if so then bulk up the materials under the rear set.
as stated my car isn't bad now - not far off my newish Mercedes at low speed. Your's is a 4 cyl, so the engine will add a bit of a droning sound, but road noise should be better with narrower tyres. Ultimately I think to get a realistic handle, either you need to take a trip in another car or you get someone whose owned these to travel with you. It's the single most frustrating job possible chasing out noise because your hearing is working against you. As you eliminate one thing, the ear just turns up the gain and you hear the next loudest thing.