I upgraded from a Ninja 300 to a white 899. I passed my MSF last summer, bought the Ninja and rode it for a few months, then borrowed a friend's BMW HP4, went "Holy crap!" Then traded in the 300 a week later (I bought the 300 outright). I put 1,000m on my 300, and put 3,500m on the 899 so far. (For me, it's a weekend/nice weather thing not a DD). Thankfully I have never downed/dropped either.
My 899 next to a buddy's 675R
Speaking of damaged fairings... Don't drop a Panigale unless you wanna pay stupid money to fix it. Unlike a 300, half the reason you buy a Panigale is to stare at it and that cracked fairing will make you sad and the sadness will make you shell out cash to fix it. Hehe
What's stupid money? Well for example the fuel tank on a Panigale cost $3,000 to replace. It's just $300 for the Ninja.
Heat's not an issue. It's more of an issue than other bikes, sure... Just don't be a squid and ride in shorts and you'll be fine.
Bonus... Yer butt feels great in the cooler fall months.
Some other issues though... There's a stupid hot start issue due to an emissions charcoal canister placed in U.S. versions, remove it (or have a shop remove it) and problem solved! The mirrors don't work well at all. Too vibratey. Also I wear earplugs when I ride any long distance... It's insanely loud stock and bonkers/ loud with Termis.
No big mods for me... I flashed mine with a Tuneboy ECU remap to add cruise control on the start button which is very nice on highway riding and I used a New Rage Cycle FE kit, which looks amazing on this bike.
Panigale are like high heels, impractical and painful to walk in, but dang do they make you look good. Lol. Bottom line there's better bikes out there in super sport, 2015 R1 is bonkers... but not prettier/better sounding bikes. And it's still very good on the twisties or on the throttle.
It's a great bike and I didn't have any trouble transitioning to 150hp from 34hp. I suggest running in Wet Mode for the first 100 miles or so, to ease into the HP jump. It cuts the HP to 100, and tones down the throttle and ups the TC/ABS. Also the 300 is garbage on the highway, so nervous! The 899? Great! Going into twisties is so much better too. It's just down to superior suspension parts vs the cheap parts on the 300.