I probably should have said that I am enjoying the thick gauges of the upper strings as I too don't like skinny steels. The synthetic D/G are certainly much chunkier than Velvet Blues and probably not far off Velvet Garbos in terms of gauge. The D and A are remarkably similar in gauge, so much so that I had to refer to the website to check the A silk colour and make sure I had them the right way round when installing (I had re-installed my Spiros before the prototype G arrived from Pirastro).
The synthetic G specifically feels very similar to me to an Oliv G in terms of smoothness and gauge but is noticeably lower tension. It doesn't sound as good/complex as an Oliv G but you wouldn't expect that (especially given the price of the Oliv!) but its not far off.
To your question, the Slap set are deeper and much less bright than Velvet Blues (which I always found had a metallic-y zing compared with other Velvets) as well as chunkier under the fingers. I really do think Pirastro could be on to something if they decide to commercialise this full set (assuming there are no issues with string longevity).
For balance, I think the downsides are (1) there is no real growl on the lower strings for those that love that particular quality (2) the E is a tiny bit thuddy and less distinct than the ADG (3) you inevitably lose a bit of a volume with the lower tension (although I think these are not dissimilar to Spiros in overall volume) and I don't think they project as well as plain gut on the upper strings. However, I think those that play bluegrass/old-time, roots or Blue Note era jazz will like these a lot. They are also VERY easy on the hands which matters to me as I have had some issues in the past (broken fingers, gout).