1. I'm not too familiar with upgraded/interchagable info on the stock synchro setup. I started blowing synchro gearboxes long ago, when the only option was going to a dog box, which is what I did. From your description however, it sounds like you definitely wore out your second gear synchros. If this is a high mileage tranny, you might try running some exotic synthetic gear lube with some kind of detergent cleaner, and change it around 2 or 3 times within 10,000 miles to try and flush out any crap which is stuck in the scynchros. More than likely, this won't work if your synchros are really worn out.
2. The clutch setup you have is made to hold the power. The higher HP goals you describe will surely blow your tranny over time. Especially with a clutch that will hold it. I've seen tons of people try to convince themselves that they can drive 'gently' enough to make the stock tranny work. The problem is that once your power level gets to certain point, it is impossible to resist the temptation of unleashing it! It only takes a few incidents of shock loading the tranny to cause damage. The tranny might not break at the instant, but the damage is already done, and failure is inevitable.