I have taken this picture with the 55-200mm Pentax DAL lens with a Raynox 250 magnifier lens attached. At very close range, I lose focus on the whole image and find it vary hard to obtain the sharpness I require. Would the Pentax smc D FA Macro 100mm f/2.8 achieve a better image than this,at this distance?
"I lose focus on the whole image"
"Would the Pentax smc D FA Macro 100mm f/2.8 achieve a better image than this, at this distance?"
No, changing lenses will not help you get more depth of field.
You can not achieve focus on the whole image because of the very narrow depth of field that comes with any high magnification photo.
Stopping down the aperture to f/16 or f/22 can help, but often even that is not enough. Many Macro photographers use special software for "Focus Stacking' which combines multiple photos with different focus points to get one photo with more depth of field.
Would the Pentax smc D FA Macro 100mm f/2.8 achieve a better image than this,at this distance?
It probably would, going just on the theory that a single purpose-built prime lens will likely do a better job than a combination of cheaper components that weren't specifically designed to work together. If you want to find out for sure, consider renting the 100mm macro for a weekend.
That said, there are other things you could do to improve your macro photography that cost a lot less. Adding light with a softbox and a flash or two would let you stop down the lens for greater depth of field. Mounting the camera on a tripod with an inexpensive macro rail would give you a more reliable focusing method.