You might consider selling the lens and get a bit better MKII version or the WR version, just to throw an alternative idea at you

Image quality they are they same but they both have their own features.
D300 will have a faster AF and number of shots a second but besides that there is not much in between them.

Patrick it's all in how they balance things.
Pentax opted to go for more details instead of smoothing out the noise, you can of course increase the noise reduction and then you get so good as the same result like the "rest".

Simply select the photo you want to edit and press develop in the top "window" next to library and slideshow.
Then on the right panel under Detail tab you will find noise reduction.
You only have two options.
Luminance, smoothens out brightness differences so to say (main noise weapon).
Color, will tone down the colour noise.

You can also just shoot RAW and pull the JPG preview out of the raw file

About lens basics link
Basicly what Mongoose just said but with pictures and some other terms
You can also find one about Depth Of Field (DOF) on the site but it's a bit more advance and you don't really need to know what is said there since it explains the science behind it.

And here is a tool to play with the focal lengths. link
As you can see it isn't linear but it's a parabola.

The difference for example between 15 and 30 is the same as between 200 and 400, they are both half of the number so therefore equal in difference. The tool above will show you that.

But then again Samsung can't force Pentax to use Pentax SMC and other coatings on Samsung branded lenses.

Then again we don't know what kind of deal they have but everything is possible like you said.

They are the same as the Pentax products.
Although I don't know about the coatings if they are the same, does anybody knows?

Then again Pentax have the cashback offer now making it £619 for the K-X with DA18-55 and DA55-300 is also a very good deal.

Pentax USA don't have this action so you will miss out.
They do however have an action with the K7 giving back $100 for every lens you buy with it at the same time. USA residence only.

My uncle has just ordered his K-X with twin kit lenses (DA50-200) for €619,-
They didn't had it in the shop so they have ordered it and he can aspect it to have it next week.

Will put it under some test if I've the change, the DFA100mm should be able to pull out all of the quality of the sensor.

Can only compare it with the K10D though but I'm sure we have some K20D users here with the DFA100.