The CF Manfrotto 055CX3 weights 3.92lb, verses the alloy 055XB at 5.07lb, or about 23% less. These are 29.2mm and 29.4mm leg diameters, respectively and are basically conparable to a Gitzo Series 3 in leg diameter.
The CF Gitzo GT3541LS weighted 3.8Lb verses 7lb for the alloy GT3340L, or about 46% less, a significant difference. These are 32mm leg diameter Gitzo Series 3.
The CF Induro CT313 weighs 4.9lb, verses 5.8LB for the alloy AT313. I think these are very comparable but I do not know the Induro line very well. These are 32mm leg diameter and sized to a Gitzo Series 3.
The above examples are as comparable as possible. In the case of the Gitzo I used the discontinued GT3541LS because the current GT3542LS has a new and much heavier (and marketed as much sturdier) mount, but the alloy version has not been updated yet, and therefore I don't think there is a current exactly comparable pair (same mount) in the Gitzo line.
Manfrotto, Induro and Gitzo are probably the most popular and widely discussed tripod lines in the case where there are exactly or closely comparable CF and alloy versions. There are some others but in previous research I recall that the results are comparable to Manfrotto/Induro (CF/Alloy ratio not as favorable as Gitzo).
You can see from the above that, in general, and historically, lower priced CF models do not have as much weight reduction over alloy as found in a Gitzo. And in the above examples, the CF Manfrotto "Series 2 equivalent" is about the same weight as the Gitzo Series 3, and the Induro CF Series 3 equivalent is a full pound heavier than the Gitzo Series 3.
In your case, with a 300mm lens, we generally recommend 32mm legs (Gitzo Series 3) for ideal performance. Some people use a Series 2 and I have done so myself, and understand the differences - a Series 2 can work, but a Series 3 is much better. For the same weight you can use a Gitzo Series 3 (ideal) or the Manfrotto "Series 2 equivalent" (not ideal), or the Induro at 1lb heavier.
Some people say that the cost of a Gitzo goes into a brand name or perhaps into long term reliability that many do not "need". But the above suggests why in this case you do get what you pay for, even at the breathtaking prices of current Gitzo offerings.
The above is an historical analysis. In the latest revisions of CF, Gitzo added about a half pound to their Systematic mounts. That makes these ratios less favorable, but presumably we get something for carrying an extra half pound and paying a couple of hundred dollars more for the privilege. But I do not own a "V2" version, nor have I handled one, so I cannot assess this.
Another Gitzo option is to replace the mount with a Markins hub. I am using one now, with my GT3541LS legs, and note that the hub weighs less than even the older Gitzo mount, and is certainly quite sturdy. So if price is no object, that is one way to keep the CF/Alloy ratio as favorable as possible. And combined with a gently used set of "V1" legs could make for a relatively economical solution, depending on the going rate for the discontinued Gitzo models.