Professional colour...why not?

Hi Aubergine,
Given your experience and knowledge of color - I don't think you have to worry about the "professional use only" warnings.
The manufacturers will not stand behind their product unless used by licensed professionals.  The color itself isn't necessarily stronger or trickier - it's that you buy the developer separately and if you use a high vol developer you'll lift more, etc.
So yes - in the big picture, what you see as a boxed kit in a drug store is usually not as strong as what you can get from a professional color.  I like professional color much better because I find the results more predictable.  A level 7N is a level 7N.  A vol 30 developer will lift 3 levels.  Period.  Some brands give you nicer color than others, but for me, using professional colors takes out a lot of the mystery.
Also - I used to work for a manufacturer of professional products - a big part of that "professional use only" message is "prestige."  They don't want to see their products show up at drug stores.  Business is better if the brand image shows exclusivity to salons.
I haven't used Topchic, but from what I could find online, it sounds like that would be an excellent choice for you!