with the la buyoff, you are only nixing the "dead" levels, in effect. with the buyoffs, it spreads all the abilities across the hd granting levels and the non-hd levels. thinking for it a bit, i would say that you should be able to buy off the same number of levels with a progression as you should be able to with the more standard la versus hd setup.
There are progressions for stuff with HUGE LA. You cannot buy off anything larger than +4, pre-epic, with the UA rules. Also, the problem with Progressions is when to buy off the LA, if you don't have a fixed level when you could do this. Normally the first level you can buy off is after triple the total LA. But if your total LA is +7, then you can only start buying off at 21+ your RHD.
The problem with Buy-off is the scale. And it doesn't change for progressions. Low LA is irrelevant, high LA is really hard to get rid of. With fixed levels this might change, although I guess they put in this funny triple multiplier for a reason. I suggest a fixed mechanism, nevertheless:
With a fixed mechanism you could buy off a level after every three levels,i.e. after 3rd, 6th, etc. You pay the cost for the next level +1000XP.
So, for instance, you take a +2 LA race and one class level or three levels of a progression containing at least one "empty" level, i.e. a level without a HD, then you wait until you have 4000XP surplus, waste them, and you are now ECL 2. Your next level costs 2000 XP, you are 2 ECL behind your companions, which means on average you get (at that level) about 10-15% more XP than them. That won't really help you catch up, much, BUT at least you'll have more open levels. However, as levels go up the benefit of -2 levels will increase to up to 50% bonus XP, IIRC.
Now I'm not sure about retroactively buying off levels. Say you become a Lich after level 11. By normal rules, you can buy off the next level 12 levels later, IIRC, which sucks. With retroactive normal rules, you buy it off at 12th, and so on. By fixed level rules, you can buy off a level at 12, 15, 18, so you won't lose all the LA before level 20. With retroactive buying off you could buy it all off pretty quickly immediately after getting it, for 4000+7000+9000 XP, and the last LA for +13000 XP after gaining level 12.