In the above illustrated combustion chamber milling the head would increase the squish. I think it's more common that the squish area is flat and at the same level that the head gasket sits on, like Tomek's Viffer head appears to be, and with those milling only increases compression ratio. Milling the cylinder block or case or using a thinner head gasket brings that flat part closer to the piston increasing squish...which might sound backwards because you're decreasing the space to increase squish. Playing with the squish is a hit miss thing, but the generic rule is that increasing squish increases performance. Kinda like increasing the compression ratio increases performance and we know that's not a hard set rule and there are limits, but in production engines there's usually room to increase both and gain power. Squish and compression can both have a pretty dramatic influence over performance, efficiency, and emissions so the factory has to do the balancing act.
Tomek does know some good shit and he has the motivation to dive in and giterdone, both are admirable. Especially the latter as I always have 57 things I should/could do that get back burnered. I think if he were as confident in his knowledge/abilities as he portrays he would engage and exchange and probably up his game by a fair bit. Whenever I meet someone who already knows it all about any field I find out that they don't and are just protecting themselves. Or maybe it's my lack of confidence that makes me exchange.....I dunno, I aint no sykologyst.