**I am only comparing vintage bikes in this reply as you can easily buy a modern bike that has huge power and is lighter than most mountain bikes. Zaddict referenced building a restomod from a vintage bike, so that is the only thing I am comparing. The zrx is an amazing motorcycle and is always the correct answer to what big bike should I buy, but it is what I consider a modern bike for the purposes of this thread**
As an owner of a slightly modified zr550 and a very heavily modified kz750, I can only say it is not a very tough choice. Going for a larger displacement bike is the only real solution as you can only make your 550 so fast.
As I am sure you remember, I have put higher compression pistons and cams from the zx550 in my Zephyr and it was a sizable upgrade in power from stock, but even if I was to bore it out to 615 it still wouldn't give me the sensation of being pushed back that the torque of my 750 provides. That is not to say my zr550 is slow....and I would wager if I raced my two bikes against each other, that the 550 could very possibly keep up with the 750. Also note, that my zr550 is geared for lower rpm's at highway speeds, so it is technically slower than if it had stock sprockets.
I have touched on this in my build thread, but one day I went and rode my 750 and 550 back-to-back and the difference in weight and power delivery is enormous!
The zr550 is so much lighter than my kz750 ( and over 50% of my kz750 is made from a zr550!) The 550 felt so light it felt like riding a bicycle in comparison. The 550 engine so effortlessly revs to redline and just gives me a sensation of bike that can't wait to take off down the road. Kind of like an eager dog pulling against the leash because it wants to play.
The 750 is a sensation of torque that propels me forward from a stand still, but once in the higher rev's it doesn't feel all that different...except it isn't as eager to scream to the redline as the 550 is.
The 750 is very easy to maneuver and does not feel lumbering in comparison to the 550 at all. But you can instantly feel the massive weight difference between the two bikes.
I have read of many instances of folks on gpz550's holding their own against the larger displacement vintage bikes. The weight differences is what equalizes the sometime huge gaps in power output.
I honestly don't know the weight difference between the 550 and 750 engines, but it is significant. I have both a spare 550 and 750 engine sitting on rolling platforms in my garage and just pushing them around you can tell the difference in heft. I lifted my completely assembled 550 motor in to the frame once but with my 750, I could only lift the bare assembled engine cases into the frame and had to install the clutch, covers, cylinder block and head while it was in the frame. I simply can't lift an assembled 750 motor off the floor. The kz900/1000/1100 motors are even bigger in size which is reflected in the sizable difference in curb weights from the 750's.
So all that gibberish was to only illustrate, that while you can easily find a larger displacement (vintage) motorcycle that will provide years of fun, you may lose a lot of the nimbleness and "happy puppy dog" feeling the 550 provides.
That why you should keep the 550 and build a really cool 650/750/900/1000/1100 restomod.
Now that my 750 project is almost done, ( just need to finish welding up the Bassani exhaust) I am now planning to remodel my zr550 with some powder coated wheels and a different exhaust. I have to keep myself busy until I can collect all the necessary parts to start the kz650 restomod I want to do.
**Totally of subject, but I need to return to your past thread about installing zx550 cams in your Zephyr. While researching the mystery Yoshimura cams I put in my 750, I instantly realized a massive chunk of mis-information I wrote in one of my replies regarding valve overlap. Needless to say is that I'm an idiot and posted some highly erroneous information that needs to be removed.**