No problem it is a very capable 2 up tourer, I have done it loads of times,
the longest trip was 800 miles in 30 Hours, because I missed the foccing boat from Rosslare to Cherbourg.
and went from Pembroke to Dover to Calais to Angers to get the holiday back on track. We were knackered the day after, but grand after that. 200 to 300 a day is enough.
You can get up to 270miles from the 02-03 FZS600 with its 22 litre tank between refills.
We have toured the around Ireland, England, France, Pyrenees and Northern Spain and on it also.
I would suggest you replace the pillion foot rest rubbers and use the same ones as the Rider foot pegs for passenger comfort. They are about £15 each, but much better for comfort and grip. Part number is 4BP-27413-00. Put a large 30mm washer between the exhaust bracket and the inside of the foot peg which reduces resonance vibration and helps comfort also.
The only other extras (besides the luggage) I have on the 600 are progressive fork springs and a slightly heavier spring on the Hagon Shock. I found the original standard spring on the Hagon too soft. Our combined weight is about 22stone with gear.
My wife agrees that the 600 pillion seat is more comfortable. (I have both and we can speak from experience) We use a bean bag to improve the 1000 seat.
Give us a call in Cork if you do the Ireland trip, lots of good places to see, visit and some great scenic good roads, as I am sure Downey will agree with