Chicken and Egg??? First it's not like there is no business development in Toronto, if taxes were too high then how/why were BAC, RBC Centre, Telus house, ICE's office component, PwC tower, etc built/planned. Toronto is still an attractive location to build an office building (note I'm sure there are a handful of similar office buildings built in NYCC and Y/E I'm simply not familiar with that part of the city. I think the equation is roughly, IF you are going to build a business centre in Toronto, THAN you are most likely going to build one in the downtown core as it is the most attractive location in the city.
Second landlords chasing cheaper taxes in the 905 region, according to you, are forced to build huge business parks with large parking because transit sucks in the 905. What comes first, the trip generators or the riders??? You can't expect a municipality to run a transit line along a currently empty (or nearly empty) road (let alone a road that may not exist yet) in hopes that it encourages developers to build transit oriented business parks. No that would be a complete waste of money. So again here we are with the chicken and the egg, developers will continue building business parks with vast parking lots because transit sucks in the 905, but 905 municipalities cannot improve transit if the ridership isn't there and that brings us back to developers building transit oriented developments.
The municipalities could require that 75% of the planned parking available at business park be underground parking so as to minimize the effect of large parking lots. However developers would squawk loudly because land is so cheap in the 905 and building underground parking is so, relatively, expensive. They could also require that the office buildings actually front onto main roads (ones that would be planned to have transit) to make it easier for transit riders to go from their stop to the office building.