I got boot floor rips last year.... The Subframe was reinforced before, but they still appeared. They were tiny rips by the spare wheel carrier. They were easy to weld closed. The main problem that you need to look for is where the rear axle carrier panel starts. On my E46 M3, the spot welds had started coming loose and the floor was dropping. We needed to weld it back up and put a brace in the rear to add more support -- why? -- because supercharger... :fencelook:
See this pic -
You can see the spot welds that let go. The metal has not cracked. The cracking you see is the rubber sealant that BMW use. The metal from the chassis legs and the rear axle carrier panel are just separating. THe rear axle carrier panel (the part that holds the subframe and diff) started dropping.
This is easily fixed by clearing away the boot rubber. Removing the diff and subframe and pushing the floor back up. Then welding the panels together and putting in some metal sheets to add some support. We then weld some brackets from the boot floor (rear axle carrier panel) onto the brace that is installed. This will help hold the floor up.
Going back to your rubber sealant on the spare wheel carrier, the rips I had were pretty much where you have rubber sealant, but don't recall seeing that much sealant. Only way to tell is to remove the rubber sealant and have a look underneath. But only do that if you are really unsure about the cracks. The spare wheel carrier is the easiest to fix. You want to check the rear axle carrier panel (as seen in my pics).