Fronts are 16.5 cm in a nasty plastic mount, and also a seperate tweater up near the door handle. Slightly harder to change but still achieveable. Need to get a speaker ring adaptor for a Corsa and modify the position of one hole. Costs about £8 a pair. This steps the speaker out of the door about an inch to match the rover fit. 17 cm will now fit. I fitted some Sony replacement separates including replacement tweaters in this way behind the sexey metal grills you get in a BRM (and not in a ZR , bit like full leather seats ) and these sound and look realy great.
I've also done a set of 17cm co-axial replacements by this method too, and just disconeccted the cheap nasty tweater.
I have metal grills over the tweeters in my zr?. Does the BRM have the tweeter in the door handle moulding or as a seperate unit as per 25/zr? I've looked for custom fit upgrades but nothing as yet -I want to put some Cerwin Vega 6 1/2 + tweeters up front - no idea if it would fit - saw a pic in the 200 Haynes manual of the mid/bass - 3 hole fitting?? strange. If the tweeter set up is the same as the BRM can I have product codes for everything what you put in.
I fear you may be correct and ZR is different to BRM. Haynes 3399 page 11.9 bottom right picture 13.3b shows 200 door handle tweater, which is same as BRM. I've had a look at MG Driven to Extremes page 18 and ZR is quite different. You're just going to have to hook the door panel off and take a look.
As you say Haynes page 12.19 shows the base mid (Rover 4 watt max, wow !) the picture does not really show how the three screw plastic mount steps the speaker out about an inch. I think there is a very high chance the ZR will be the same. I think the tweater might be the same too just mounted differently.
I fitted Sony XS-HA 1724 (150W ) these have a silver metalic cone in the centre of the mid range which is visable through the silver metalic mesh BRM grille - tasty
For the ultimate rear it has to be a stealth shelf from Acoustic Solutions, the best £90 you can spend. Check their website, see the thread on bass box.