Well I've come from seven years in the automotive industry, specialising in selling engine parts to reconditioners and rebuilders around Perth.
In all that time I cannot think of a single motor for which throwing a rod could be considered 'common'. This kind of thing will generally only happen to an engine that has been severely mistreated or abused: ie ran low on fluids or over-reved (probably from down-changing as suggested, as most late model vehicles are fitted with rev-limiters).
Based on requestes for engine parts I used to get, I regard some of the Nissan and Mazda engines to be darn near bullet-proof! SR20's, RBxx's, later B6 variants, and in particular the later BP engines I virtually never sold parts for!
Compare this with the engines that kept many reconditioners in business - Mitsi 4G32, 4G62, 4G63, 4G5x's, 6G72, Toyota 3SFE, 5SFE, Holden/Opel 16LE, 18LF (and any Daewoo variants!), Holden V6, Falcon 6's, etc, etc...I don't even want to talk about some of the Nissan and Toyota diesels!! ;)
Admittedly, while a few of these engines can certainly be considered over-represented in the market (4G54's for example have been in various forms in Sigma's, Magna's, Triton's, Pajero's, Canter's from the late 70's through to the early 2000's, the fact remains that after a few years you develop a soft spot for certain engines from certain manufacturers - and the later Mazda BP's are one for me.
Am I concerned about the liklihood of my stock SP throwing a rod during 100,000k of 'spirited' driving with the odd track day? Not in the slightest! I'd be more concerned for the life of the turbo over that length of time...
Keep it in good knick, service and replace the fluids regularly and I highly, highly doubt you would ever have a problem with the bottom end.
Having said that...I agree entirely that if you were chasing HP figures, then make the investment in at least pistons and rods. Remember, unlike the SE (which will definitely have different rods and pistons), the SP is of course essentially a stock BP with a turbo bolted on, so it lacks the preferred lower compression pistons usually seen in this application and you're never going to see 20+ pounds of boost like some of the WRX drivers!
I would tend agree that a reliable 160-180rwkw would probably be around the most you could hope for from a stock SP, just by modifying boost/intake/exhaust/fuel/timing. Any more than that and I'd say you would simply be pushing too much boost and be unable to create a reliable tune. This of course, will induce engine knocking and pinging and result in damage to the motor, although I'd still say you're far more likely to put a hole in a piston, or crack some ring lands before you risk putting a rod through the block.
Now if you completely screw up your water injection and manage to hydraulic lock the engine, then you might throw a rod...
