BoBO’s right on, most significant gains in power start from inside out with big-bore high-comp pistons. Along with bigger Mikuni carbs and hours of dyno-tuning to tie it all together. Nothing you can bolt on will make as big difference. Let me make analogy...
1. Bolt-on mods such as air filters, jet-kits and full exhaust with dyno-tuning will give you +5hp. You’ll notice difference for $2k. It’s like taking your gal to spa for pro hair and makeup job.
2. Engine build with big-bore high-comp pistons, bigger valves, port & polish (along with all above). For $4k you get +15hp. This is like giving your girl conplete face job with new eyes, ears, nose, lips and chin-tuck. Boob-job, liposuction, rib-removal and years of gym workouts with the best Olympic trainers. Before and after comparison is supermodel BAWOWZERS!!!
From number of times you’ve asked this question, you’ll really want Ninja 400. Almost twice power of your 250 and performance is BAWOWZERS better!
Let’s put this in direct street perspective. 34-yr old 250 does 0-60 in about 7s with 15.3s 1/4-mile. That means you can keep up with modern Corolla in drag race.
Ninja 400 does 0-60 in 4.6s and 1/4-mile in 13.1s @ +103mph. That’s keeping up with supercars like Corvette, Porsches, new Supra! Ninja 400 has 34-yrs of upgrades. Imagine what you can do to your 250 if you had 34-yrs and unlimited money to upgrade it!
If you drag-race your 250 against 400, you will fall behind from very beginning. They both weigh same, but 400 had close to twice as much power. By time you cross finish line 1/4-mile away, 400 will be more than entire football field ahead!
Best move for you in terms of least amount of time, effort and money is to sell your 250 and SV650 to buy Ninja 400. It’ll be lighter and faster than both and you’ll be extremely happy with it.
BTW, not just bikes, hock everything you’ve got to buy 400. TVs, cars, furniture, computers, fridge, wife & kids, break into parent’s retirement fund, etc. Ninja 400 is THAT good and worth it.