I had a SRX 600 which was a pig too - almost the same engine. A 'Splitfire' spark plug did help a lot, and also improved the starting on my XTZ660. Checking the carb and airfilter wouldn't do any harm either.
The engine issue sounds like a slipping clutch, assuming that is happening on hills or when you increase the engine load by accelerating, etc ?
Tyres... are you sure you are on a 50/50 ? I doubt it. It sounds like you are using the bike for normal type use and there are plenty of good options recommended by the tyre companies. These will be dual purpose trailie tyres - we will all have our own preferences and you can search this forum for lots of different views on each brand. None will be 50 profile, I suspect....
Honda CBR 1100 XX, Yamaha XTZ 660, Harris Special