I've been offered to buy a set of 911 front struts for converting my 4 bolt to 5 bolt.
The guy selling also has the 911 crossmember and A-arms for sale.

What's the pro's/cons of swapping everything to 911 spec?

Better selection of torsion bars ...

I considered using the one from my '84 Carrera (parting out) but the ATE 23mm master cylinder is almost too long as it is, and I didn't want to take the chance. Someone with both will know for sure.

I'll admit the aluminum piece looks better... for something I never look at. Don't know the weight difference. Anybody weigh them recently enough to remember?

You will need the aluminum cross member to clear the 23 mm master cylinder if it's the Mercedes style this I know for sure. I ran this setup in my 71 914 years ago. I have both a 23 mm Mercedes and 23 mm master that automotion sells. I can take some measurements of each if needed. I've seen the steel cross member used/modified by putting a nice dent in it to clear the master cylinder. I chose not to do this and went with the aluminum cross member. If you go with the 911 a-arms check to see if they are the late (under car) or early (through body) sway bar style. Either one can be used in a 914 but if you use the late style you will need the sway bar as well. I got a set of late style a-arms and cut the sway bar mounts off. I ended up welding on the U tabs to use a through chassis sway bar.

I run a 19mm master cylinder with Carrera front calipers/rotors. It works very well...

Based on what little that I do know- I really don't know much, I would buy / bolt the whole package on. If you did that you will get 1) 5 bolt hubs, newer strut. 2) have a thicker rotor..think it went up another 2mm thick, 3) alum rack mount...and use the new style under body sway bar. ...there's a 19mm and a late style 20 /22 mm one. No welding of tabs on A arms, and no holes to cut in body.....a great set up.
Oh bigger brakes too