As a brief reply:

3D Printing of pieces as big as bumpers, dashboards, car doors, ... in a single go is possible. Materialise (manufacturing site in Belgium) has SLA printers which can print (roughly) up to 2 x 1 x 0.5 m. The resin can be polished by hand after printing, so even high gloss paint jobs are possible.

In terms of strength: most SLA parts would suffer from high temperatures (summer sun) and/or big forces (wind friction at high speed, for longer durations). The printed parts (based on the original CAD files) would probably be strong enough for display purposes, but not for actual driving. Reinforcing the inside of the part with glass fiber would make the part stronger and possibly usable to test drive a car (this has actually already been done by manufacturers to support their design phases). Using the SLA-printed part as model for (silicone) moulds would also be possible, but I'd doubt you'd be able to properly fill the mould...

Last comment: if you're thinking of actually printing a bumper -> expect a quote between 10.000-15.000 Euro (+ shipping)... You'll need to make very exclusive items to make it interesting to potential buyers :-)

Best regards,
Rikky