The term hopback, or hopjack, has different meanings to different brewers. Before the advent of pelletized hops and hop extracts all brewers used whole hops. Hopbacks were used primarily to strain hops from wort after wort boiling. There were two basic designs for hopbacks; batch and continuous designs. The batch-sized hopbacks looked similar to mash tuns and were designed to hold the contents of one brew. Basically, the kettle was drained (or “knocked-out”) and the hopback acted as a strainer to remove the hops while the wort flowed through the strainer installed in the bottom of the hopback. Continuous hopjacks were designed with screw augers that moved the spent hops out of the hopback while wort flowed through the strainer and out. Both basic designs have changed little over the last century and are still used by brewers who use whole hops
Brewers who really liked beers with pronounced hop aroma figured out that if a batch-style hopback was loaded up with hops before the kettle knock-out the resulting beer would be full of wonderful hop aromatics, but not hop bitterness. This is true because hop oils, the aroma constituent of hops, are quickly extracted by hot wort while hop bitterness requires hops to be exposed to boiling for a period of time to isomerize alpha acids into their more bitter and soluble cousins, the iso-alpha-acids. The use of the hopback in this fashion is not too different than a drip-style coffee maker or the bed of juniper boughs used in Sahti brewing.
As a brewer and beer connoisseur, I think the three methods you address in your question, late hopping, hopback hopping and dry hopping, are all distinctively different in how hop aroma is expressed in the finished beer. Late hop additions, especially when pellet hops are added to the kettle and not removed from wort until wort cooling is complete, frequently add bitterness to beer in addition to aroma. This feature of late hop additions can be avoided if whole hops are used and then separated with a hopback. As you suggest you can accomplish a similar effect by using a hop bag and removing the bag of pellets after boiling. Late hopping and hopback hopping both extract aroma compounds from hops using hot wort while dry hopping uses beer to extract hop aroma. This makes dry hopping very different, at least to my sensitive schnoz, from these hot wort treatments.
OK, now for the subjective part of the answer. I think hopback hopping is a great substitute for late kettle additions. As far as the aroma contribution is concerned you may find that the aroma retention is better with the hopback since adding hops to the kettle does result in aroma loss if the boil extends much past the addition. However, you may end up with a slightly less bitter beer with the hopback. A slight increase to the first hop addition is an easy way to tweak any recipe you have that you want to reproduce with your new hopback.
In the “old days” of really hoppy beers, you know like in the early 1990s, simply hopback hopping or dry hopping with a pretty generous dose of hops, such as 1⁄2-ounce per gallon or roughly 4 grams per liter, was enough to make most beer drinkers say “wow, this is a really hoppy beer.” Things are different nowadays, especially since that documentary, “Hopheads Gone Wild,” inspired the ultra-hoppy tipple movement. Some of these beers use the belt-and-suspenders approach to brewing and are hopped using as many methods as possible. There are really no rules, as long as you are willing to pay to play. There is no doubt that if you hopback hop and then later dry hop the same beer that another layer of hoppy complexity will be added.
I am a fan of balance, even with big beers that are often a bit unbalanced by their very nature. Some-times it is difficult to appreciate the nuances of a technique when it is combined with others. It’s kind of like attempting to appreciate an ethereal jazz flute solo while the drummer is bashing on the kit with sticks. But if you really want to crank the dial up on the hop-o-meter the combination of hopback hopping and dry hopping can be fun. My inclination would be to balance the two methods such that neither overwhelms the other. In my experience I find that it takes about twice the weight of hops used in dry hopping to approximately equal the impact of one part of hops when hopback hopping, for example use a 1⁄2 ounce per gallon of dry hops and a 1⁄4 ounce per gallon of hopback hops. With that being said, you bought a tool, so go experiment with it and figure out what works best for your preferences!