Since I work at a bike shop, it is my job to try to get people to spend their money. But I do also try to tell people (politely and gently) if I think they are wasting their money. A customer who finds out (later) they've been taken for a ride (pun intended), is a customer who will not return.
But if I outline to them their options, their final decision is at least an informed one (and they can't blame me!).
Ask yourself the question: "What would it cost me to replace this bike with the current equivalent if it were stolen or destroyed?". Another subtly different question (provided you're thoroughly interested in the sport) would be: "What am I prepared to spend on my next bike?"
Once you have a ballpark figure in your head you can then start figuring out for yourself whether any upgrade is really worth the outlay compared to the cost of an entire updated bike. Also bear in mind that, within reason, updating an entire bike (including the frame) will provide a different level of performance, whereas upgrading individual components provide, at best, subtle improvements.
I gather that the Specialized Secteur is a relatively current 8 speed roadie. Where I work a bike like that is worth AU$800-1000 (depending on manufacturer). To get a proper wheel upgrade (ie in terms of performance AND durability) on the cheap, you're looking at a set of custom built wheels (from a choice of spokes, rims and hubs) at a cost of around AU$600 minimum (new).
"Upgrading" to a cheap set of fancy spoked proprietary wheels (retailing in the AU$400-700 range), is no upgrade at all. They will either be just as heavy as what you have now (if they were any lighter they'd be more fragile) and they will likely cost you more to service in the future.
To get a proper upgrade with a set of proprietary wheels will cost you around 150+% of what your entire bike is worth. These sorts of wheels are designed for racing and not for everyday riding (and will again bring with them an increased future servicing cost).
In short I don't think you can do much to make your bike faster without spending an inordinate amount of money. The biggest difference will come from your training (as Peter has already said).
But I would only recomend a coach if you need to be motivated (or kicked up the ****) by someone to do your exercise. These days the technical aspects of exercise are well documented, so if you can read, you can coach yourself. Joe Friels The Cyclist's Training Bible. is a good starting point. From here you can get as technical/untechnical as you want. The most important thing to do is to just ride your bike.
Learning to work on your own machine is also a good way to save money and I too would recommend Lennard Zinn's books.
