There is no way to transfer toolbar settings between SU versions. When I install a new version of SketchUp I make a screen shot of the old version and import it as an image into the new version. That allows me to see my toolbar arrangement without switching between versions.
The new versions of SketchUp use a different toolbar manager and it's actually easier and faster to display the toolbars you wish to show. You can also create customized toolbars for the native tools which means you can omit tools you don't use anyway. For example i have no used for the toolbar buttons for Orbit and Pan so I left those off. I also don't use the Dimension and Text tools in SketchUp because i do all that stuff in LayOut. Leaving those tools off gives me more screen real estate for other tools I do use.
If you created a customized template (style, dimensions camera position, etc. Open a new session in the old version of SketchUp. Immediately save the file somewhere like your desktop. Then open the new version of SketchUp, open that file you just saved and immediately go to File and choose save as Template. Give it a name, make sure Use as Default is checked, hit Save and bob's yer uncle.
If you want help creating custom toolbars, drop me a PM and i'll help you out.
Edit: By the way, if you had plugins installed in SU8, do not just copy them over to the SU2015. Install fresh copies of them. With SU2013 or maybe it was SU2014, they had to make a switch from the old, unsupported Ruby 1.8 to Ruby 2.0. This changed required many plugin authors to update their scripts. Use the automatic installation methods to install new plugins.
Also see this:
http://www.finewoodworking.com/item/117133/making-sketchup-your-own
