I've never swapped seats before but i have serious doubts that a manufacturer as competent and efficient as Honda would change the bolt pattern in the car just for a batch of Si's coming down the assembly line. You could probably verify the Si seats would match up by contacting a dealer's parts department. They normally have detailed diagrams and would be able to compare the bolt patter between the seats. Good luck!
you can fit almost any seat from any honda, into your honda all you will need is the proper brackets to hold it down good ....ask your local dealership!
if you want to get aftermarket seats, some do come with brackets/rails