Hi Steven:
There's a whole contortious line of reasoning that leads up to it, but I favor requiring a min raise here to 14 total.
And it's not spelled out in any specific rule, but to me under Rule 1, "complete" always implies "I intend to increase the bet" (the closest analog of which is "I intend to raise" from the official terms of poker). This is more important in my calculus than the exact amounts involved ("all in for 6", "okay I make it 8" (erroneously)).
But at the end of the day, there is no exact translation of "complete" here so it's ultimately a Rule 1 decision IMO. To me the best interest of the game comes down to "How can I enforce as much discipline as possible here?". And in this case, I favor construing something tantamount to a raise to being a raise when it doesn't exactly apply.... If the bettor mumbles about it, he (and everyone else at the table), will be more cautious about their betting discipline in the future.
Because it's Rule 1, there's at least one and probably more opposite cases that can be made. One is that saying complete is numerically "8". What if the guy had SILENTLY pushed out a 5 and three 1s? It's less than 50% of the way to a full raise of 14 (which would be 10 total as either a 5 and five 1's or ten 1's), so that would be a call if just going by the chips pushed. But he didn't silently push out the 8, he declared something that says "i'm going to put more chips out". A) I don't want people having a shot they can take to test the tables interest in a raise and then smooth call and B) to me "raise" is implied anytime you verbalize something greater than a call amount.
However it was discussed in a few casual breakout groups before and at Summit VI whether to put such a question on the agenda. I must say the majority of people I heard from favor treating a declared amount exactly as they would treat the same amount of chips pushed silently, i.e. treat verbalizing "8" here exactly as you would pushing out 8 in chips silently. And the reasoning isn't so much that declaring 8 doesn't indicate an intent to raise, but rather it's procedurally easier for a dealer to remember to treat them the same. Your OP is in a grey area between the two, I think, where the player has gone beyond declaring a higher number that is less than an amount to raise; he is actually declaring a process of increasing the total bet. So he's put himself in this wobbly area and to really make people think twice about what they say I favor ruling a full raise to let people know they should take their betting seriously. It's not exactly the same, but distantly akin to the TDA language that says "I bet the pot is not a valid bet but it binds the player to making a bet, and if declared while facing a bet it binds the player to making a raise", because we don't want the guy taking a shot with "I bet the pot" then turning around and calling. But ultimately until something is put down in writing, it's a Rule 1 decision IMO that could be ruled either way (call or full raise), and I respect either. My least favorite thing here is to give the guy a choice: "sir you can't complete here, so would you like to call or raise"? Just like I wouldn't give him a choice if he declared "I bet the pot".