I really think that once you get a good feel on your party and their "combat capacity", the regular rules will work really well.
As you said, putting tougher fights is, to a large extent, very similar to having more fights. In addition, careful use of terrain and tactics can make a fight tougher even without altering the XP budget.
I think that using fewer tough fights will be just fine with the basic system. There are only two things that I would note:
Use more threats, not higher level threats. Monsters substantially more powerful than the PCs just don't 'work right' in the fight. They are too hard to affect, and too effective.
Don't telegraph which days are going to be 1 encounter days, or 2 encounter days, or 3 encounter days. If the PCs KNOW that they can use all of their daily powers in this fight, they ARE going to whip them all out. If they aren't sure, they will probably save them until the fight gets hard. If 5 pcs each blow a daily in each of the first 4 rounds of combat, the fight is probably over at the top of turn 5.
The other thing to think about is not a problem, but just a consideration. Planning on fewer tougher fights can discourage certain character concepts, and encourage others. The high damage, high risk striker with fewer surges looks pretty good if he knows he can plan on 3 fights a day. 6-9 surges will cover him for three fights, in most cases. The durable defender with 15+ surges isn't probably going to use 5 per fight, so he's got wasted abilities. Make sure that your players are thinking about your changes to the 'game assumptions' and be prepared to allow them to tweak their characters a bit (extra retraining, perhaps) if things don't work out as planned.