With modern computer controlled engines, heat range has become much less of a concern. Using the OEM heat range will be perfect from "old lady" to mildly agressive street use(the kind that does not get you locked up...) and in-between. If you are racing often, the colder plugs are probably a good idea.
When switching brands, you may find that the heat ranges don't exactly corespond, and this is true of the Autolite, vs an AC plug. I ended up going one step colder with the Autolite, as I was getting some pinging with the recommended range.
As far as platinums not giveing more power, well I've never really seen any plug do that. If the cars I worked on were less of a royal biotch to swap plugs in, I'd probably be swapping standard plugs every 15k, the way I did when I had cars that the plugs were easy to get to, but where a standard plug has a change interval of 30k or 15k for an anal person like me, a stainless body platinum has a 100k or 50k change for me. In the case of my car, the OEM plug is a stainless bodied platinum, but Autolite has the same plug for 1/2 the price and it works as well as the OEM plug so why spend money on OEM? Or you could look at the fact that I had to experiment to find the right range, so going back with the same plug would have been easier...
As far as ignition systems, many fall for the bigger is better philosophy, and unless you are running a substantially modified engine, OEM will be hard to beat. An exception is the newer multi-spark ignition systems, that do show gains in economy and driveability as well as some slight power gains, especially at lower and midrange, but again, not really that cost effective.
I tend to upgrade wires when I replace them, but in my case, it's pretty hard to beat the OEM wire. You can get an equivalent wire for a lower price, or a slight upgrade for about the same price, but again, will you notice the difference on a stock or mildly modded engine? Probably not, at least if you are truthful with yourself.
I think people put way to much thought into ignition systems. OEM is usually PDG, and will be reliable to boot. Putting aftermarket coils and ignition systems in, tends to reduce reliability, although I have to say, its still a lot better than OEM from a couple of decades back. It's when you start getting deeper into the mod game that you NEED to look at the ignition again.