Ian,
I can't comment on the cap as my setup is different, but as for changing the leads, I would say it's probably a good precaution after that length of time, and Magnacor offer an excellent product, and great after-sales service if you ever have a problem.
Yes, the leads do just push in, but a couple of things to check
1)It's surprisingly easy to get the leads mixed up, so change them one at a time and label them - I use cable ties to denote 1,2,3,4 from the front of the engine at BOTH ends of the leads
2) The leads have boots at the end of them, and it is quite easy to think that the connectors are well engaged, when instead, it is just the boots holding the leads on. If you pull the boots up the cable a bit, you can ensure that the connectors are well engaged, the push the boots down.
I don't believe that top quality leads like Magnacor will ever add power, but poor leads can certainly make problems and rob you of some.
Regards,
Michael.