I have experience machining (actually grinding) hard chrome and electroless nickel plating. Electroless nickel is actually tougher to grind and hold a tolerance with than hard chrome. Personally, I think Thundertiger engines are a very good value, but the quality of their piston-to-sleeve fit is often slightly lower than that of OS for the simple fact that Thundertiger's nickel plating is thicker. It appears that the TT engines require more finish grinding than the OS engines and this will cause issues with the roundness of the cylinder bore. Nickel also has a tendency to work harden if you push it too hard. The more you "rub" on it the harder it gets. For more information about chrome and nickel plated cylinders see my website:

http://sparkysbarn.com/motodynamo/chromenickel.htm