The stock sleeves should survive most of what you can throw at them and still be functional as a street car (IE sub 350-400 or double the stock output). How long they will survive a bad tune or a hotspot that cause detonation is the problem. I think the factory rods will probably fail first in that situation and travel through the sleeves destroying both but still.

I've personally never been big on "slide in" block guards. For one thing, the location is horrible. Where is the one place in your engine where the most heat is generated and you want coolant as close to as possible, the top of the sleeves and bottom of the head. A block guard will occupy that space keeping coolant away. I've seen sleeving companies that actually "close the deck" or pin the new sleeves to the block an inch or so down from the top deck of the block and I liked those. Another issue is expansion. Most guards are made of the same material as the actual block itself (aluminum) and will expand at a simliar rate but being as close to the combustion chamber as it, it will most likely get heat first and expand faster if not more than the rest of block. This could put compresion forces on the top of the sleeves themselves and cause problems. I've never built and engine with one and I'm sure there are people who have used them and had success but I've personally never been a big fan.