Kobo has been my second biggest market after Amazon for years now.
Their promos are hit and miss for me. Some do well, others don't. For example, the recent 3 books for 5 bucks promo in the UK, Australia and New Zealand I was in did nothing, but then I couldn't even find the promo link on the respective sites. I still keep applying to any Kobo promo that's a good fit for my books, because it can't hurt and might just do something.
I haven't noticed any impact from the Walmart thing at all. Most of my Kobo sales are outside the US anyway (Canada usually, but also Australia, New Zealand, the UK, Germany, Switzerland, Belgium and the Netherlands) and I have seen no increase in US sales at Kobo.