A sealed system mean not open to the atmosphere so no open header tank in the loft,,so no frost risk ,no build up of gunge in the open tank which can be sucked into the system,no chance of oxygen being pulled into the system causing rust,and pressure gauge lets you know if the system is water tight,
Another thing is that most red expansion vessels are usually shot by 8 to 10 years,maybe do a pressure test on that when system is down.
All boilers are suitable for micro bore,,the pumps are all good for any pipe size.