I've said it before and I'll say it again, if it were as easy and inexpensive to effectively upgrade the Sigma/SD Series of pistols as these aftermarket parts manufacturers suggest, then Smith & Wesson would have upgraded the design with similar parts of their own by now or otherwise licensed out these companies to produce these parts for S&W.
The reality of the situation is this, the Sigma/SD Series is a budget line of pistols which is already just about as good as it gets for the price, and attempts to significantly upgrade, enhance, or improve on the design with simple drop-in aftermarket parts simply cannot succeed.
In other words, the Sigma/SD Series will never be an M&P, even if you match or exceed the cost of a M&P in aftermarket parts. Sure, you can improve it marginally with better sights, wider takedown levers, and other such minor accessories, but replacing the trigger, the guide rod, and the RSA aren't going to do much. (In fact, steel guide rods are actually known to compromise the structural integrity of the frame, making them utterly detrimental.)