Certainly tempting to upgrade to ATP turbos, it would lower the boost needed to create the same power. OTOH, if you are not looking to get into "big" power, a tune + 3 bar MAP + colder TStat has been the way to go for most people.
I would highly recommend getting maintenance up to date first tho to make your decision easier. Spark plugs, turbo piping, intercooler, PTU/RDU fluid, trans fluid, coolant (prob not for your flex), brake fluid, u name it, all due/overdue at this mileage. If you do decide to "upgrade", use the Special Operating Conditions/Severe Duty schedule for maintenance.