I was not convinced about this conclustion & so , decided to some work myself on current top 100 players & this is the result -
Atp ranking Name ITF junior high rankings
1 Djokovic, Novak (SRB) 24
2 Nadal, Rafael (ESP) 145
3 Federer, Roger (SUI) 1
4 Murray, Andy (GBR) 2
5 Ferrer, David (ESP) None
6 Soderling, Robin (SWE) 2
7 Berdych, Tomas (CZE) 6
8 Tsonga, Jo-Wilfried (FRA) 2
9 Fish, Mardy (USA) 14
10 Monfils, Gael (FRA) 1
11 Almagro, Nicolas (ESP) 18
12 Simon, Gilles (FRA) 242
13 Tipsarevic, Janko (SRB) 1
14 Del Potro, Juan Martin (ARG) 3
15 Roddick, Andy (USA) 1
16 Troicki, Viktor (SRB) 10
17 Gasquet, Richard (FRA) 1
18 Dolgopolov, Alexandr (UKR) 21
19 Wawrinka, Stanislas (SUI) 7
20 Melzer, Jurgen (AUT) 26
21 Isner, John (USA) 93
22 Cilic, Marin (CRO) 1
23 Mayer, Florian (GER) 8
24 Verdasco, Fernando (ESP) 294
25 Ljubicic, Ivan (CRO) 11
26 Stepanek, Radek (CZE) 42
27 Monaco, Juan (ARG) 61
28 Lopez, Feliciano (ESP) 46
29 Chela, Juan Ignacio (ARG) 34
30 Anderson, Kevin (RSA) 28
31 Raonic, Milos (CAN) 35
32 Dodig, Ivan (CRO) 376
33 Llodra, Michael (FRA) 41
34 Youzhny, Mikhail (RUS) 20
35 Bellucci, Thomaz (BRA) 15
36 Granollers, Marcel (ESP) 302
37 Davydenko, Nikolay (RUS) 510
38 Bogomolov Jr., Alex (USA) 57
39 Fognini, Fabio (ITA) 8
40 Haase, Robin (NED) 3
41 Tursunov, Dmitry (RUS) 246
42 Muller, Gilles (LUX) 1
43 Seppi, Andreas (ITA) 68
44 Young, Donald (USA) 1
45 Andujar, Pablo (ESP) 5
46 Robredo, Tommy (ESP) 8
47 Nishikori, Kei (JPN) 7
48 Giraldo, Santiago (COL) 84
49 Tomic, Bernard (AUS) 2
50 Malisse, Xavier (BEL) 10
51 Gulbis, Ernests (LAT) 525
52 Starace, Potito (ITA) 530
53 Garcia-Lopez, Guillermo (ESP) None
54 Kohlschreiber, Philipp (GER) 24
55 Baghdatis, Marcos (CYP) 1
56 Montanes, Albert (ESP) 530
57 Nalbandian, David (ARG) 3
58 Kubot, Lukasz (POL) 30
59 Kunitsyn, Igor (RUS) 61
60 Machado, Rui (POR) 350
61 Karlovic, Ivo (CRO) 417
62 Istomin, Denis (UZB) 412
63 Kukushkin, Mikhail (KAZ) None
64 Benneteau, Julien (FRA) 17
65 Berlocq, Carlos (ARG) 306
66 Stakhovsky, Sergiy (UKR) 49
67 Petzschner, Philipp (GER) 8
68 Blake, James (USA) 92
69 Ferrero, Juan Carlos (ESP) 17
70 Sweeting, Ryan (USA) 2
71 Nieminen, Jarkko (FIN) 11
72 Dimitrov, Grigor (BUL) 1
73 Volandri, Filippo (ITA) 41
74 Lu, Yen-Hsun (TPE) 3
75 Riba, Pere (ESP) 868
76 Beck, Karol (SVK) 24
77 Ramos, Albert (ESP) 9
78 Rosol, Lukas (CZE) 50
79 Harrison, Ryan (USA) 7
80 Hanescu, Victor (ROU) 96
81 Rochus, Olivier (BEL) 11
82 Mahut, Nicolas (FRA) 3
83 Cipolla, Flavio (ITA) None
84 Devvarman, Somdev (IND) 40
85 Haider-Maurer, Andreas (AUT) 9
86 Falla, Alejandro (COL) 4
87 Bachinger, Matthias (GER) 146
88 Gimeno-Traver, Daniel (ESP) 4
89 Mannarino, Adrian (FRA) 303
90 Prodon, Eric (FRA) 3
91 Klizan, Martin (SVK) 1
92 Kavcic, Blaz (SLO) 68
93 Junqueira, Diego (ARG) None
94 Sela, Dudi (ISR) 9
95 Russell, Michael (USA) 36
96 Kamke, Tobias (GER) 642
97 Capdeville, Paul (CHI) 13
98 Berrer, Michael (GER) None
99 Darcis, Steve (BEL) 8
100 Gil, Frederico (POR) 10
If you take ITF top 100 as the criteria for determining if these players played ITF juniors seriously , than there are 25 players in this list who are outside that limit. So, it seems 1 out 4 players did not take junior circuit seriously among top 100 players. This can be extended further for players ranked between 101-200 but I do not think the % will vary drastically.
If you look at the countries which make up those 25 players, Spain seems to be the dominant one. It has 6 such players out of total 14 top 100 players & the % goes close to 50%. I think this Spanish model is something which out TNTA folks are trying to replicate.
Also the multiplier effect from junior to senior rankings appears mcuh lower with an average of around 8 in these top 100 players.