You can call any object method directly from a browse_record instance, no need to reach the pool. Method parameters cr, uid and context will be passed automatically. To get the display name for your example:

    sale_order_line.product_id.name_get()[0][1]

Note that name_get returns a list of pairs: [(id,name),...]

If you really need to have the value as a field! you can create a function field that uses name_get() to get the value.