Hi Blackbeard,
Welcome to brewing, your life is about to change
Playing around with recipes can be fun and get some good results and isn't that spreadsheet great? I'd encourage
anyone to tinker with it, especially the new brewer. I still use it and find it a massively helpful tool.
It's true that the recipe probably isn't 100% true to style for a steam beer and this is because it's using a pre-made kit as the base. These recipes are lovingly referred to as 'Kit & Bits' which essentially means that you are just altering and enhancing a simple inexpensive beer kit to taste like something much better than it usually might with the help of a few easy additions.
The clever boffins at Coopers have created this recipe to be pretty damn close to the genuine article in taste and appearance but they also want to keep it simple enough for everyone to make which is probably why it isn't spot of for a califoria common.
Whether you should make the extra additions is completely up to you, I think the changes it will make are minimal but I'm also a bit lazy. The dextrose will up your alcohol level in the beer, thinning out the body a little to be closer to the style and the DME will alter the colour a little (though I think the difference 200g of DME will make will be pretty minimal) but by all means give it a crack.
Some extra effort and you enjoy the taste (and bragging rights) of something as close to a california steam beer as you could personally make it. Otherwise you'll just have a beer that tastes very nearly the same but might not have been so much fun to make without your own personal touch.
Do whatever is going to make you happier sir.