The LCR0170 situation is slightly involved, so you indeed want to approach things eyes wide open. The first tidbit to know is that this frame is sold as two versions, LCR017-D ('integrated handlebar') and LCR017S-D ('S' for 'stem only'). The reason I mention this is because the spacer configuration (and hence stack) differs between the two versions. I am attaching two pictures from LightCarbon's website for ease of reference.
LCR017-D (with integrated handlebar)
If you look at the LCR017-D picture, you'll see something that they call 'bottom cover' (usually called 'headset cover') that sits above the top bearing and the C-shaped clamp. This cover is a mandatory piece which adds to the stack height. Guesstimating from the pictures it's about 12mm tall; maybe someone on here has documented the exact height, or you can ask LightCarbon.
Quick aside: most bikes need a headset cover, so that's not special to LightCarbon. The height of the cover varies by manufacturer/model, however.
Important note: the HBR17 handlebar that comes with this frame has an (unusual) +8mm lift, increasing stack by that 8mm. (Most handlebars are neutral, meaning they don't have any lift.)
LCR017S-D (stem only)
The LCR017S-D diagram shows two covers, the bottom cover and the top one; I believe these are the mandatory spacers you mention. The top one looks quite chunky; maybe that's the 15mm you mention.
And yes, assuming these numbers (12mm for the bottom cover + 15mm for the top cover) you'd see your reach decreased by about 1cm (8.8mm according to what my computer tells me). But if you are worried about reach you can go with a longer stem.
LCR017-D vs. LCR017S-DOverall, I think the minimal stack height on the LCR017-D will be a little lower since you don't have to deal with the top cover; you do get that +8mm rise from the handlebars though. And as discussed, the top cover effectively decreases reach. These numbers don't mean much, however, without taking into account any of the additional regular spacers you may or may not want.
AlternativesTake a look at the Long Ten LTK266-D; size XS: stack 506, reach 374. It's a 'bring your own stem+handlebar' deal, so you have plenty of options for how you want to set up the front end of your bike.
Potential heartbreak: there is a lightweight version of the LTK266-D, the LTK266-D--SL. But I don't see a lightweight size XS in the geometry chart, plus the charts between the versions don't agree - hopefully someone can shed some light on what's going on here.
a) regular version:
https://www.ltbikes.com/index/products/view/id/211.htmlb) lightweight version:
https://www.ltbikes.com/index/products/view/id/421.htmlc) several threads on the forum