^^^No you're confusing 350 Mission. WonderlandPark is referring to 535 Mission (Boston Properties, 27 floors, 304,000 SF) and 222 Second (Tishman Speyer, 26 floors, 350,000 SF).
The floorplates for both towers are going to be small (10-13,000 SF). I believe the marketing effort is not for large users with 100,000+ SF commitments on the market, or tech firms, which prefer large open floorplates, but rather smaller users for which there is no new class A space available in the city. Either way we are talking about 650,000 SF, which is only slightly larger than the recently redone 680 Folsom building, which by itself is 521,000 SF and over 85% leased to just two tenants who needed large contiguous blocks. I had heard 222 wasn't going to happen until Foundry III was leased. Then Google was circling Foundry, but then pulled back. Not that Foundry III with its large floorplates (23,000-35,000 SF) is direct competition with 222, both being developed by the same firm. I have heard another large user is circling Foundry III.
What will kill TBT is not the current market, but rather the cost of developing the building. It's not like TBT is going to get full service rents in the $70s while 535 Mission next door gets $60s. But maybe it will require preleasing in the $70s to justify construction. At least Boston Properties is very patient money