I think the set up that you already have actually works very well for a typical digital photography workflow. Lightroom is made especially for digital photographers, and its modules reflect the step-by-step process you would use for organizing, developing, and presenting your photos.
Photomatix Pro is a program that is specific for HDR, and it does HDR very well. Photoshop CS5 also does HDR, and many reviewers have said that its HDR rivals or comes close to that from Photomatix. If you already have Photomatix Pro, I'm not sure I would throw that away just to get Photoshop CS5.
Topaz Adjust does wonderfully colorful and creative adjustments to your photos, and you see many examples of photos that have been retouched with Topaz Adjust here. Although I personally have not used Topaz Adjust much, I'm sure that if you sat down and took the time, you could probably figure out how to re-create those richly colorful adjustments in Photoshop CS5. However, the whole point of using Topaz Adjust is that you can do all those beautiful adjustments much much much quicker using the plug-in, rather than manually trying to figure out how to redo all those adjustments in Photoshop CS5.
As photo_chick mentioned above, Lightroom uses the exact same RAW processing engine that Photoshop CS5 uses. All the sliders that you see in Lightroom are also present in Adobe Camera RAW (which is found in Photoshop CS5 and Photoshop elements).
Although technically you can choose to use Photoshop CS5 alone to re-create the workflow and postprocessing tasks that Lightroom, Photomatix Pro, and Topaz Adjust does with your current lineup, I would argue that your overall workflow would be TONS slower if you only used Photoshop CS5 to do your work. Plus, you would not get Lightroom's excellent "Library" module to quickly organize, caption, tag, and keyword the thousands and thousands of photos in your library.
Unless there are specific features in Photoshop CS5 that you really really need, I would probably stick with what you already have. As many people have mentioned, photographers who use Lightroom tend to use it for about 90-95% of their postprocessing needs. I'm not sure that using Photoshop CS5 alone by itself would improve your overall workflow (it may even, in fact, slow you down), but it would help address the rare 5% (maybe up to 10%) postprocessing tasks for your photos.